OK everybody, here is an update. I have now spoken to Len Haggis, and he thinks that the problem that I am having is more likely to be due to wear on the teeth of the quadrant (67-3347) so I have ordered one of those and the two gear shift selector forks. They have to come out to gain access to the quadrant so while I am at it I will change them anyway for peace of mind. The total cost is £45 that is for to new selector forks (£15 each) and the quadrant also £15. As a matter of interest the selector forks that were offered on Ebay were priced at $99 (£78) for just one, plus shipping and taxes. Quite a big saving by buying from Len Haggis! Thank you all for the interest you have shown, and for your help, and suggestions.
John