The problem is not the combobox but your not-shown-here code that expects a value from the combobox.

Apriumben:

@gustav you certainly put me on the right track! whilst looking for this non-existent code (not shown!)

I discovered that the query 'qryRecentBookings' referenced a different form and I had in fact previously created a new query that referenced this form, but hadn't applied it to this combo box.

The two query names were quite similar but I clearly should have read the error message more intently. I have a feeling I previously copied a form used for another purpose, rejigged the bits I needed to but didn't check this particular combo was working. Thank you for making me recheck everything!