It depends Steve..... If I was overpaid by a small company and I liked the people and management there I would declare the over payment...(always)
If I was overpaid by a small company and I disliked the people and management and working conditions and pay etc there,I would (and have before) screw them and laugh after a few months after the account department still did not discover the error.
If I was working for a Government office/dept and they overpaid me.I would do exactly what you have done and put the money aside..After that I would knock em!!
The UK has become a dog eat dog country and Honesty doesn't always pay these days IMO...
That said,if you or your Mrs feel guilty about it then declare it.... Its not worth the sleepless nights... Again.... Only IMO.
