This offer was supposed to end yesterday (Tues 2nd) but they've extended it until Sun 7th.
The same deal as always and applies to both online and instore purchase. Spend over £50 and get a £5 gift voucher. Spend £100 or more and get a £10 gift voucher. Note, you do not get both £5 and £10 for having spend over £100, just the 1 x £10 :)
The gift vouchers expire on Feb 14th 2009 so you've got a reasonable amount of time to use them.
Full terms and conditions can be seen in the deal link so click it.