The Road Vehicles (Construction and Use) Regulations 1986 s.39 as amended (see link below)

All fuel tanks:

• Shall be constructed and maintained so that the leakage of any liquid from the tank is adequately prevented.
  
  
• Shall be constructed and maintained so that the leakage of vapour from the tank is adequately prevented.
  
  
• If fitted to a vehicle first used on or after 1st July 1973:

                   - Made only of metal.

                   - Fixed in such a position and so maintained as to be reasonably secure from damage.

• A two-wheeled motorcycle (with or without a side-car) first used on or after 1st February 1993 need not have a fuel tank made only of metal.

The Road Vehicles (Construction and Use) (Amendment) (No. 8) Regulations 1992