Prads We used to do a similar thing with chemical tankers, which was based on risk assessment... Only guidance reference I have ever found is in HSG 76 - Warehouse & Stotage para 513 parking. Just general comments really but says 'Vehicles and trailers should only be left on a surface suitable to support the full weight of the vehicle/trailer and any load etc.'. INDG 312 says very similar words... I think I may have some other stuff tucked away somewhere..

Not safety details, but environmentally, if you are using it for storage, you will need to treat it as a storage tank - i.e. secondary containment, no possibility of entering drains etc. Environment Agency will frown upon the use of a vehicle for storage on a permanent basis, but on a temporary basis would look for spill mitigation and pollution prevention.