I bought a Jaz tank for my car. My chassis is not true Haynes Roadster, but close. They are available in the UK. It is probably a little more than you want to spend, but I believe it is a much safer tank, especially knowing where the tank sits!
Circle Track Tank Capacity: 10 Gallon
Circle Track Dimensions (LxWxD): 24" wide x 9" front to back x 12" tall
• SEAMLESS CROSS-LINKED POLYETHYLENE OUTER SHELL,
• MIL SPEC B-83054 AVIATION FOAM FILLED,
• D-RING BAIL HANDLE CAP ASSEMBLY WITH CHECK VALVE VENT,
• MIL SPEC ANODIZED AN-8 PICKUP AND VENT FITTING,
• PICKUP HOSE ROUTED TO RIGHT REAR CORNER,
• 360° MOUNTING STRAP RECESSES.
http://www.rallydesign.co.uk/product...ucts_id=184 3
Note: I think the 10 gallon capacity is US gallons, your gallons may vary!