Thanks to Peter and Bob for the responses - the problem was that I missed checking the primary key field as updateable. There was a check in the view designer in front of the primary key but it needed another check to make it updateable.

I added the error message checking as suggested but it wasn't much help - it stated that the update failed because nulls were not allowed in the field. I wasn't storing nulls, I was trying to update a non-updateable field.

Thanks again - it was reassuring to know I was on the right track and had a good possibility of success!

Pat