Thanks for that wirehopper, however whilst I may have misunderstood - that article seems to suggest that the Access GUID is still essentially an autonumber which would mean that I would not be able to explicitly copy an existing one into the field. This is essentially my primary requirement. I need a unique ID that can be overwritten with a different value, which autonumbers do not permit. It looks to me like the GUID would suffer from the same problem.
Thanks
Mike