diff --git a/src/app/features/tech-record/components/tech-record-summary/tech-record-summary.component.ts b/src/app/features/tech-record/components/tech-record-summary/tech-record-summary.component.ts
index ce4a5a9a79..bc658d661c 100644
--- a/src/app/features/tech-record/components/tech-record-summary/tech-record-summary.component.ts
+++ b/src/app/features/tech-record/components/tech-record-summary/tech-record-summary.component.ts
@@ -156,7 +156,10 @@ export class TechRecordSummaryComponent implements OnInit, OnDestroy, AfterViewI
 		});
 
 		this.form.valueChanges.pipe(takeUntil(this.destroy$)).subscribe((changes) => {
-			this.handleFormState(changes);
+			// prevent merging of array of objects - always override
+			const isArray = (a: unknown, b: unknown) => (Array.isArray(a) ? b : undefined);
+			this.techRecordCalculated = mergeWith(cloneDeep(this.techRecordCalculated), changes, isArray);
+			this.technicalRecordService.updateEditingTechRecord(this.techRecordCalculated as TechRecordType<'put'>);
 		});
 	}
 
diff --git a/src/app/forms/components/autocomplete/__tests__/autocomplete.component.spec.ts b/src/app/forms/components/autocomplete/__tests__/autocomplete.component.spec.ts
index c1b6e98d38..4a365c58d8 100644
--- a/src/app/forms/components/autocomplete/__tests__/autocomplete.component.spec.ts
+++ b/src/app/forms/components/autocomplete/__tests__/autocomplete.component.spec.ts
@@ -80,15 +80,15 @@ describe('AutocompleteComponent', () => {
 		expect(control?.touched).toBeTruthy();
 	});
 
-	it('should propagate "" to form control when input is left empty', () => {
+	it('should propagate null and reset to form control when input is cleared', () => {
 		const findOptionValueSpy = jest.spyOn(autocompleteComponent, 'findOptionValue');
 		const control = component.form.get('foo');
 
 		autocompleteComponent.handleChange({ target: { value: '' } } as unknown as Event);
 
 		expect(findOptionValueSpy).toHaveBeenCalled();
-		expect(control?.value).toBe('');
-		expect(control?.touched).toBeTruthy();
+		expect(control?.value).toBeNull(); // use null to indicate the field is empty
+		expect(control?.touched).toBe(false);
 	});
 
 	it('should propagate "[INVALID_OPTION]" to form control when value is not an option', () => {
diff --git a/src/app/forms/components/autocomplete/autocomplete.component.ts b/src/app/forms/components/autocomplete/autocomplete.component.ts
index 1effb9968d..5004987d5a 100644
--- a/src/app/forms/components/autocomplete/autocomplete.component.ts
+++ b/src/app/forms/components/autocomplete/autocomplete.component.ts
@@ -88,6 +88,13 @@ export class AutocompleteComponent extends BaseControlComponent implements After
 	handleChangeForOption(value: string) {
 		const optionValue = this.findOptionValue(value);
 
+		// Handle case where input is cleared fully, so it returns to default state without triggering validation
+		if (optionValue === '') {
+			this.control?.patchValue(null);
+			this.control?.markAsPristine();
+			return;
+		}
+
 		this.control?.patchValue(optionValue ?? '[INVALID_OPTION]');
 		this.control?.markAsTouched();
 		this.control?.updateValueAndValidity();
diff --git a/src/app/forms/custom-sections/adr-section/adr-section-edit/adr-section-edit.component.ts b/src/app/forms/custom-sections/adr-section/adr-section-edit/adr-section-edit.component.ts
index 3e42d175a0..ce24d7b648 100644
--- a/src/app/forms/custom-sections/adr-section/adr-section-edit/adr-section-edit.component.ts
+++ b/src/app/forms/custom-sections/adr-section/adr-section-edit/adr-section-edit.component.ts
@@ -1,6 +1,6 @@
 import { ViewportScroller } from '@angular/common';
 import { Component, OnDestroy, OnInit, inject, input } from '@angular/core';
-import { ControlContainer, FormBuilder, FormGroup } from '@angular/forms';
+import { ControlContainer, FormBuilder, FormControl, FormGroup } from '@angular/forms';
 import { ActivatedRoute, Router } from '@angular/router';
 import { ADRAdditionalNotesNumber } from '@dvsa/cvs-type-definitions/types/v3/tech-record/enums/adrAdditionalNotesNumber.enum.js';
 import { ADRBodyDeclarationTypes } from '@dvsa/cvs-type-definitions/types/v3/tech-record/enums/adrBodyDeclarationType.enum.js';
@@ -131,12 +131,8 @@ export class AdrSectionEditComponent implements OnInit, OnDestroy {
 				'Reference number or UN number 1 is required when selecting Product List'
 			),
 		]),
-		techRecord_adrDetails_tank_tankDetails_tankStatement_productListUnNo: this.fb.array(
-			[
-				this.fb.control<string | null>(null, [
-					this.commonValidators.maxLength(1500, 'UN number 1 must be less than or equal to 1500 characters'),
-				]),
-			],
+		techRecord_adrDetails_tank_tankDetails_tankStatement_productListUnNo: this.fb.array<FormControl<string | null>>(
+			[],
 			[
 				this.adrValidators.requiresAllUnNumbersToBePopulated(),
 				this.adrValidators.requiresAUnNumberOrReferenceNumber(
@@ -312,9 +308,30 @@ export class AdrSectionEditComponent implements OnInit, OnDestroy {
 	}
 
 	handleInitialiseUNNumbers() {
-		this.techRecord()?.techRecord_adrDetails_tank_tankDetails_tankStatement_productListUnNo?.forEach(() =>
-			this.addUNNumber()
-		);
+		const unNumbers = this.techRecord()?.techRecord_adrDetails_tank_tankDetails_tankStatement_productListUnNo;
+
+		// If there are un numbers, then prepopulate them
+		if (Array.isArray(unNumbers) && unNumbers.length > 0) {
+			unNumbers?.forEach((number, index) => {
+				this.form.controls.techRecord_adrDetails_tank_tankDetails_tankStatement_productListUnNo.push(
+					this.fb.control<string | null>(number, [
+						this.commonValidators.maxLength(
+							1500,
+							`UN number ${index + 1} must be less than or equal to 1500 characters`
+						),
+					])
+				);
+			});
+		}
+
+		// Otherwise, add a single empty UN number
+		else {
+			this.form.controls.techRecord_adrDetails_tank_tankDetails_tankStatement_productListUnNo.push(
+				this.fb.control<string | null>(null, [
+					this.commonValidators.maxLength(1500, 'UN number 1 must be less than or equal to 1500 characters'),
+				])
+			);
+		}
 	}
 
 	handleInitialiseSubsequentTankInspections() {
diff --git a/src/app/forms/custom-sections/vehicle-section/vehicle-section-edit/vehicle-section-edit.component.html b/src/app/forms/custom-sections/vehicle-section/vehicle-section-edit/vehicle-section-edit.component.html
index a17f097b77..eebabdbae4 100644
--- a/src/app/forms/custom-sections/vehicle-section/vehicle-section-edit/vehicle-section-edit.component.html
+++ b/src/app/forms/custom-sections/vehicle-section/vehicle-section-edit/vehicle-section-edit.component.html
@@ -132,7 +132,7 @@
     label="Vehicle subclass"
     formControlName="techRecord_vehicleSubclass"
     [options]="VEHICLE_SUBCLASS_OPTIONS"
-    [tags]="[{ label: TagTypeLabels.REQUIRED, colour: TagType.RED }]"
+    [tags]="getVehicleType() !== VehicleTypes.SMALL_TRL ? [{ label: TagTypeLabels.REQUIRED, colour: TagType.RED }] : []"
   ></govuk-checkbox-group>
 
   <!--  Coupling type (TRL Only) -->
@@ -301,6 +301,7 @@ <h1>
       formControlName="techRecord_euVehicleCategory"
       [width]="FormNodeWidth.S"
       [options]="EUCategoryOptions"
+      [tags]="getVehicleType() === VehicleTypes.SMALL_TRL ? [{ label: TagTypeLabels.REQUIRED, colour: TagType.RED }] : []"
     ></govuk-form-group-select>
   </ng-template>
 
diff --git a/src/app/forms/custom-sections/vehicle-section/vehicle-section-edit/vehicle-section-edit.component.ts b/src/app/forms/custom-sections/vehicle-section/vehicle-section-edit/vehicle-section-edit.component.ts
index e4fd8c68cb..017ae8edec 100644
--- a/src/app/forms/custom-sections/vehicle-section/vehicle-section-edit/vehicle-section-edit.component.ts
+++ b/src/app/forms/custom-sections/vehicle-section/vehicle-section-edit/vehicle-section-edit.component.ts
@@ -29,6 +29,7 @@ import {
 	MONTHS,
 	PSV_EU_VEHICLE_CATEGORY_OPTIONS,
 	PSV_VEHICLE_CLASS_DESCRIPTION_OPTIONS,
+	SMALL_TRL_EU_VEHICLE_CATEGORY_OPTIONS,
 	SUSPENSION_TYRE_OPTIONS,
 	TRL_VEHICLE_CLASS_DESCRIPTION_OPTIONS,
 	TRL_VEHICLE_CONFIGURATION_OPTIONS,
@@ -318,6 +319,8 @@ export class VehicleSectionEditComponent implements OnInit, OnDestroy {
 				return HGV_EU_VEHICLE_CATEGORY_OPTIONS;
 			case VehicleTypes.PSV:
 				return PSV_EU_VEHICLE_CATEGORY_OPTIONS;
+			case VehicleTypes.SMALL_TRL:
+				return SMALL_TRL_EU_VEHICLE_CATEGORY_OPTIONS;
 			default:
 				return ALL_EU_VEHICLE_CATEGORY_OPTIONS;
 		}
diff --git a/src/app/forms/custom-sections/vehicle-section/vehicle-section-view/vehicle-section-view.component.html b/src/app/forms/custom-sections/vehicle-section/vehicle-section-view/vehicle-section-view.component.html
index 203ea9fa80..df5c0e161e 100644
--- a/src/app/forms/custom-sections/vehicle-section/vehicle-section-view/vehicle-section-view.component.html
+++ b/src/app/forms/custom-sections/vehicle-section/vehicle-section-view/vehicle-section-view.component.html
@@ -30,10 +30,9 @@
         {{ vm.techRecord_manufactureYear | defaultNullOrEmpty }}
       </dd>
     </div>
-    <ng-container *ngIf="
-      vm.techRecord_vehicleType === VehicleTypes.TRL
-    ">
-      <div class="govuk-summary-list__row">
+    <ng-container *ngIf="vm.techRecord_vehicleType === VehicleTypes.TRL && vm.techRecord_euVehicleCategory !== 'o1' && vm.techRecord_euVehicleCategory !== 'o2'">
+
+    <div class="govuk-summary-list__row">
         <dt class="govuk-summary-list__key">Date of first use</dt>
         <dd class="govuk-summary-list__value" id="test-techRecord_firstUseDate">
           {{ vm.techRecord_firstUseDate | date: 'dd/MM/yyyy' | defaultNullOrEmpty }}
@@ -229,6 +228,7 @@
         </dd>
       </div>
     </ng-container>
+
     <ng-container *ngIf="
       vm.techRecord_vehicleType === VehicleTypes.HGV ||
       (vm.techRecord_vehicleType === VehicleTypes.TRL &&
@@ -247,8 +247,7 @@
       </div>
     </ng-container>
 
-  </dl>
-    <dl *ngIf="vm.techRecord_vehicleType === VehicleTypes.PSV" class="govuk-summary-list">
+    <ng-container *ngIf="vm.techRecord_vehicleType === VehicleTypes.PSV" class="govuk-summary-list">
       <div class="govuk-summary-list__row">
         <h3>
           Seats:
@@ -308,5 +307,6 @@ <h3>
           {{ vm.techRecord_alterationMarker | defaultNullOrEmpty }}
         </dd>
       </div>
-    </dl>
+    </ng-container>
+  </dl>
 </ng-container>
diff --git a/src/app/models/options.model.ts b/src/app/models/options.model.ts
index aeff2a8d56..0da4e42d58 100644
--- a/src/app/models/options.model.ts
+++ b/src/app/models/options.model.ts
@@ -35,6 +35,11 @@ export const HGV_EU_VEHICLE_CATEGORY_OPTIONS: MultiOptions = getOptionsFromEnum(
 
 export const PSV_EU_VEHICLE_CATEGORY_OPTIONS: MultiOptions = getOptionsFromEnum(PSVCategories);
 
+export const SMALL_TRL_EU_VEHICLE_CATEGORY_OPTIONS: MultiOptions = [
+	{ label: 'O1', value: EUVehicleCategory.O1 },
+	{ label: 'O2', value: EUVehicleCategory.O2 },
+];
+
 export const ALL_EU_VEHICLE_CATEGORY_OPTIONS: MultiOptions = getOptionsFromEnum(EUVehicleCategory);
 
 export const HGV_VEHICLE_CLASS_DESCRIPTION_OPTIONS: MultiOptions = [