Continuing story................
To answer a previous observation.... turning the wheel to the right at a right hand junction in town - then no problem - the amount of turn the wheel requires can be as much as one complete revolution. The lever cancels every time.
Turning to the left - similar amount of wheel movement - and the problem presents.
From memory the wheel has a protruding tab close to the splined mounting hole. I suspect the problem may lie here, with wear/age being the root cause.
Have secured another steering wheel (Ebay again!!) which I'm assured cancelled an indicator both ways.
Here's hoping!!
M.