Just went in the Currys in Cheltenham and picked up an Apple Mighty Mouse which was on clearance at £24.97.
It appears to be reduced because Apple has changed the branding on these mice to "Apple Mouse", removing the "Mighty" tagline. They had the ones in the new boxes at £39, but these old stock ones were only £24.97, despite the fact that they are exactly the same product.
I took the last one in the Cheltenham store, and it is possible that there are more of them around, although quite possibly not as this is a DSGi .97 priced item.
Good luck with finding them! Hope this is of use to someone :) Will post a photo of the receipt below...