Since my last post I have had a chance to try the bike out. The cork gasket that I fitted has been a great improvement although not a complete cure, and the reason for this is that the diameter of the gasket is slightly undersize. This is the reason that the exercise has not been a complete success. The gasket tends to displace itself to one side as you swing the cap to the closed position, and because the neck rim is so narrow when this happens the pressure on the gasket is not evenly distributed. I intend to go to Hobby Craft and see if I can find a piece of cork from which I can fabricate a gasket that is a snug fit in the cap to prevent it from moving out of position. The 1/8" thickness seems to be about right. I do not think that cork will be a permanent long term solution though, because eventually the petrol will degrade it. I think that a PTFE gasket will probably be the best solution to this problem. I have also fitted a piece of tube to the tank breather pipe and this has worked well in catching spillage from this source. I did notice that fuel also escapes from the small hole in the top of the cap. Not much that can be done about that, but the amount that gets out from this source is minimal. So, there are three separate sources for the fuel to escape. Based on my experience with the cork gasket that I fitted, and the fitting of the breather pipe tube I am now pretty sure that the spillage problem can be resolved.
John