Well, that's very odd! This time it worked, and I don't know why as I don't think anything had changed.
Anyway, here is exactly what I did:
Go to A5 Driver Assistance->Long Coding->Configuration for lane departure warning KI15->change to Last Setting
Then drag the green blob across to write the change and it was successful.
Switching off and restarting with Lane Assist disabled has proved that the setting has been changed correctly.
I also changed 'Lane Assist System Mode' to 'Steering Wheel Vibration not active', just to be sure. Hopefully I will no longer find the steering wheel trying to tug me towards oncoming vehicles on narrow country roads!!!
There were two other things I discovered while browsing through other items which I found fascinating - in control unit 22 - AWD there is a setting for 'Diff Lock' which was set to Not Installed. I don't know if this refers to the mechanical diff lock (VAQ) or the electronic diff lock (XDS), but on the Alltrack the XDS should be installed. I wonder if this setting is incorrect for my car, as I have to say that I have previously suspected that it is not active because I find in fast tight cornering I don't get the feeling of the front end actively stopping the understeer like I did with my previous Octavia 4x4. If anything the opposite. I have set this to 'Installed' and it will be interesting to see if it makes a difference.
Also in 44 Steering Assistance there is a setting for 'All Wheel Drive (AWD)' and that also was set to 'Not Installed'. Once again I have successfully changed this to 'Installed' so it will be fascinating to see if I can feel any difference. I know that one shouldn't really change two things at once, but I can always change one back and see what the effect is then.