Thank you for your always helpful responses. I've not use the 'Bars' feature but the snap to grid has always been fine for me in the past.
Anyway, I tried again on a new song and it worked. Puzzled at why, I changed the resolution on the screen so that more bars appeared and I eventually got to a point at where I clicked and the time cursor snapped to one quarter before the measure that I had clicked. If I lower the resolution again so that less bars appear then I can get it to work.
I can manage with this, but I wonder if others get this same problem.