Solved it!!
The column that was causign the problem was AuditStatus. This is part of a composite key. It turns out I set the composite key up (incorrectly for my needs) as being unique. The update I was trying to do would have created a duplicate key.
Quite why the error message given out was so misleading I don't know, but at least it's sorted.
Thanks for your feedback Darren
Rgds
Reg