I got home early yesterday, so inspected my reader in the daylight. I couldn't see anything wrong with the operation of the page turn buttons. There was a nice sounding click and they felt as if they had worked. I couldn't see any dust accumulation etc. So I decided I'd try a reset as advised (using the pen to press the reset switch as described in the manual).
This seems to have cured the problem! How strange! So could it be some sort of software error?
I normally power down the device when I have finished reading to maximise the battery life. (Or am I kidding myself?).
So, there must be something different about resetting to power down. Maybe resetting does some re-initialisation of some non volatile memory.
Anyway, I'm happy again now. For a while.