Just an update to this to say how I fixed it - in case anyone else in the future faces the same problem.
I discovered that I had too many deleted items still held on the system (maybe the 'Automatic management' setting isn't working? ) and so went through them all one at a time and used the permanent delete button until the deleted section was empty. Then reset box by unplugging at wall and re-booting after a few minutes and switching on again - hey presto !! everything back to normal and recordings re-established.
Plan to do a clear-out of the deleted items manually on a regular basis from now on to prevent it happening again (fingers crossed).
