Thanks Dave,
I can confirm that all rubbers are and were in place when I removed the tank. Could the tank have been changed after the damage maybe?
I intend to ask the p/o re this, he lives in the same village as I do.
Interestingly, he may have some photos of trips he has undertaken, apparently, my bike has been around the Nurburgring, travelled to the arctic circle in Norway and travelled extensively around Europe two up!!!!
I'm going to do some extra research as I have heard from others in the village that at some point in the past (60's & early 70's) he competed in trials with Don Smith and Martin Lampkin!!!!