Thanks RJ..... have just tried what you suggested - Lever up (right turn) little white tab pops out. Push it up with screwdriver and lever cancels. Did the same to the left, tab pops out and cancels when pushed down with screwdriver.
Soused the mechanism with WD40, let it sit and wiped it clean. Worked just the same.
The little white nylon(?) tab does not appear to be unevenly worn - but I do not know what a brand new one looked like.
Looks like I may have to take the steering wheel off (again) and see what engages the white tabs as the wheel is moved.
Another steering wheel?

? Might this be possible answer?? or can some mod be made to existing wheel to ensure the white tabs make contact and release the lever?
Incidentally, the replacement stalks have all worked properly - for a short time - then each has become u/s on left turns as they have been used.
This is becoming an obsession...!!
M