Chris,

Thanks for the welcome.

I've got underneath it, and the tank is approximately 62 x 29 x 20. Assuming the tank is flat on top, I make that almost 36 litres. First time I filled it, I tried to brim it, and poured about three or four quid's worth of super unleaded all over the forecourt before I realised I'd obviously hit the breather hole. Second time, I chickened out and only put 20 litres in it.

Some tank dipping and more testing required, I reckon.