At ScrewFix's Trade Counters, if you make any purchase then we give you a flyer with two promotions - one of these is 10% off for you and 10% off for a mate when you spend £50 or more between July 1st and July 11th. I certainly give one with every purchase - whether it's £1 or £100. I'm not too sure if codes can be redeemed online however.
The other offer is if you spend £500 online between those dates, then you get a free sat-nav.
You just have to write your customer number on the back (which is displayed on every invoice) and then your friend's customer number on theirs - and the code is valid for use between July 1st - 11th at every counter (as far as I know).