If you need a tank let me know i have one for sale.

A Dowty seal, is a washer with a rubber lip/seal type affair in the center hole, as you tighten the bolt onto it the rubber forms a seal, As Skids said tho a copper washer is more than enough, Afterall they are good enough for brakes under pressure.
glyn

Thanks for info guys, I'll try a copper washer and the right bolt first of all, that'll hopefully sort it if not I've got the options of a dowty seal and/or tank fixing/replacing.

Cheers