Mastertronic seem to be celebrating some sort of anniversary and as a result have a competition you can enter. When you enter it, you automatically get a 20% off code. However, it seems that the code is NOT unique so you do not need to enter the competition to get money off.
So, you can get 20% OFF your final amount on any boxed (PC/DS/PS2 or Wii) or from their download PC store.
Seems pretty cool and doesn't look like it has a price limit (i.e. spend £10 get 20% off or spend £100 and get 20% off etc..).
PROMO CODE: MM874
Ends until the end of March.