Don't know if this will still work, but anyway:
1) Buy £100 argos gift card and get £10 free
2) Preferably go to another store and buy the following:
Black DS - 568/1857
DS Lite Accessory Kit - 567/8688
The Incredibles Game - 568/1668
Scooby Doo Game - 568/0030
Masterplug 15m Extension Kit - 552/0248 (or something cheap to take the total over £100)
Total for the above items comes to £101.94
Use the £100 gift vouchers to pay and £1.94 of your cash.
Get another £10 voucher free.
Total of £20 of vouchers and a DS with a game and accessory kit for £101.94