Tesco in-store have reduced the price of some of their XBOX games. I noticed in particular that Guitar Hero 4 has been reduced to just £12.00. Next cheapest I can see is game online is at £20.00 so seems like a good deal to me if you already have controllers from a previous one!
Asda did have this for £10.00 a while back but may have been store specific so if you live near both then try ringing Asda first
Do note though that if you buy the new one certain bundles give you this game free but if you already have controllers this is a great deal