I don't know if this will work as not tried & tested...
But looking as this "Anything you can do, we can do better" war between Asda & Tesco, it may do us poor customers' a favour!
It's all based on "comparable shopping" ????
But for those that do not understand the concept, here goes!
If you were to buy a box of 160 PG Tips in Tesco, they would cost you £4.25. When you get home and check your receipt on-line, it may be possible (if they are included in the comparables') to have purchased the exact same item in Asda for (currently) £3.00 - thus £1.25 cheaper in Asda. Under the Double the Difference offer, this may give you a return of £2.50, but like I say - I haven't checked this yet!
It might be worth noting that if this doesn't work, or you can't be bothered with whole receipts thing, they are currently on offer in Sainsburys until 15/03/2011 for £2.15!