Is there any sort of documentation or certification which would legally be required for a large tool chest installed in a vehicle in place of the rear seats? Also, what documentation would you be asking for to ensure the safety of such a retrofitted option?

What type of vehicle (car, small van, medium van)? In retro-fitting are you impacting any manufacturers installed devices subject to MOT inspection e.g. operation of rear seat-belts? What kind of weight is the toolbox and contents (will its addition exceed the vehicles designed carrying load)? How is the toolbox secured to stop it slamming in to the back of the driver? How are the contents secured to stop them sliding side to side affecting vehicle stability? How is the toolbox fastened to stop items flying round the cabin in the event of a collision?

The vehicles are 4x4 trucks, rear seatbelts have been removed as part of the install as there are no longer rear seats. The retro-fit has been conducted by the manufacturer but it isn't a standard option and has been custom built for the company requirements. Box is secured at the base bolted to the floor where the rear seats would previously have been. It is fully enclosed with lockable doors on the sides to prevent anything moving within the cab. The top and front are fully enclosed. The box does come with a manufacturers weight limit and this won't take it over the vehicles carrying limit. My concern is that as it isn't a standard option I feel there should still be some certification or documentation around the construction and installation to show that it has been done safely and poses no risk to the driver or passengers.

Agree with Ron at #3, you can modify vehicle as per your requirements, having regard for the construction & use regulations. You may consider https://www.gov.uk/vehic...vidual-vehicle-approval.

Thank you all for your comments and suggestions.