Thanks Ian. I've managed to buy one on e-bay for £19.95 now, which seems a good price as most people are selling them for over £30. I'm now wondering if perhaps there isn't a seal where I was thinking there should be one ! I'll come and say hello and pick your brain, if I may

I want to get the tank and mudguards transferred to no 1 bike then give your mate a call about the dented tank.
I'll be looking for a suitable o-ring for the petcock seal while I'm at Stafford - I can only find them listed for sale in the USA which seems a bit odd.