Just received an email about the H&M sale and it states:
Our Christmas gift to you!
Spend instore between 27th - 31st December 2011 and we will give you £5 off your next purchase. Simply keep your receipt with the £5 offer and redeem instore between 2nd – 15th January 2012 only.
To redeem a minimum of a £5 spend applies and the offer cannot be used towards the purchase of gift cards.