I tried all those things Richard, but each time I got the same result :-(

However, I did find out what was causing the problem, so if anyone else experiences it, this is how I found and fixed the problem.

After much searching, (of both the database and the internet with no success), I looked at the table (which is the control source for the form in the back end) to see if that had become corrupt; but no, it was fine.  So then I went back to the front end and checked to make sure that the link to the table was still there.

Of course, it was, so I double clicked it to open it and got an error message saying that Access couldn't find the object!  But the table was shown as linked?  And I knew it was there in the back end and was fine.

So I deleted the link and then re-linked it and, bingo, working exactly as it should.

I'm not at all sure how or why the link became corrupt as I hadn't worked on that part of the program for months, (a user had reported it).

Also, I have to say, that Microsoft really ought to be providing more meaningful error message by now given the 'maturity' of the product.  If I can double click on a link and get an error message that the object isn't available then surely MS should be providing the same error message when the form tries to open it.

Then again, it is Microsoft we're talking about  ;-)