Thanks all for your suggestions as to what might cause my problem. All looked at and evaluated and found as follows:-
The tracking is good, checked it twice. The steering damper was not all that good at damping, or not damping. On inspection the steering damper anchor plate was fitted miss-aligned, which caused the damper to be only part effective. Shimming between the anchor plate and the frame sorted that out. The front fork legs went up/down very freely and had been drained of oil and re-filled with 7.5 fluid ozs of 20SAE oil, the l/h leg oil seal leaks very slightly but nothing to worry about. The steering head bearings are free, they swing easily when the bike is jacked up to lift the front wheel clear and no, their is no back/forward movement when pulling to and fro. Being a swinging arm model there is no play in the swinging arm bushes.
So. I rode 6 mile locally yesterday, everything as noted above, and it still didn't want to turn corners. It was fine hands off in a straight line but trying to swing left/right using body weight only was a definite no-no, hands on bars and turn, turn, turn.
My first ride was 18 miles on A roads at 35-45mph and 10 miles of fell roads at 20-30mph. Yesterdays 6 mile ride was on A roads at 40-50mph. I can only conclude that it can only be tyres
Now, my tyres are correctly inflated but I have not a clue as to how old they are and I cannot see any date mark on either of them. The front Avon Speedmaster 3.25 x 19 has AC1 impressed on one side of the tyre and E 23155 raised lettering underneath the MADE IN ENGLAND mark on the opposite side of the tyre. The rear tyre is a Dunlop Gold Seal 3.50 x 19 and it has 25318 in raised numbers under the DUNLOP mark on one side of the tyre and 25302 in raised numbers under the DUNLOP mark on the other side. XKB is impressed on one side of the tyre only,
So---I have no idea how old these tyres are, have you? I always thought that tyre ages were a 4 figure number. The first two being the week number and the last two the year number. So 0219 is the second week of 2019. But this doesn't work with the numbers that are on my tyres. Can anyone throw any light on the subject? Are they that old?
Obviously I am going to have to bight the bullet and replace the tyres but if you can think of a reason why my bike is misbehaving I might be able to put it off a while??