Try the following:
Delete the field from the layout. Use the field tool or field picker to put it back and reset the various triggers and formatting.
Create a brand new "from scratch layout". Use the field tool or picker to set up the field on the new layout.
On each, test to see if this corrects the issue.
Many thanks for your reply. I will try your suggestions.
BTW what I did neglect to say was these files are FM11 conversions still using the 'classic' theme. Am trying to get client to commit to a complete overhaul of the UI, it's looking very tired, maybe a good lever!
Deleted the field from the layout and put it back. No problems since Monday.
Much appreciated, Thanks.