Not sure whether this was a one-off (or where to post this item), but I returned 3 wall-lights to Homebase today. They were originally on a 3-for-2 offer but I had lost the receipt. The helpful lady on the Customer Services counter said I could exchange the lights for something of the same (full price) value.
So basically, I originally bought three £15 lights (for £30), took them back and was allowed to spend £45 on other stuff. Not completely honest I know but I needed the extra £15 more than they did.
Who says you should always keep your receipt?!?