Right bear with me on this one.
I tried this today and it works. You do not technically save molah as you still have to spend your savings at Tesco, but if you shop there regularly it won't be a problem.
Fairy Excel Non Bio Gel is currently £4.52 in Tesco. Thus costs £9.04 for two bottles.
Asda are currently selling the same at £6.00 for two.
Tesco's are running their "Double the Difference" guarantee.
So after buying these you enter your receipt details on the Tesco's Double the Difference Guarantee website and end up getting a voucher for £6.04 - thus technically costing you £2.96 for the two bottles.