Just bought myself a shiny new DS (£100) with the Brand New Guitar Hero Game (£35) and essentials kit (£4 on sale from £10), which total £139 separately, for £99 in Zavvi, Cambridge (no reason why not nationwide!).
This is part of their standard promotion for DS+any game+essentials kit for £110, but paying with a Virgin Credit Card makes it £99 (10% off).
INSTORE ONLY NOT ONLINE!!
By the way the Guitar Hero Game includes the hardware for playing the game, it is not just the cartridge. I noticed instore that they are selling an official DS Guitar hero bundle for £130 in Zavvi (£125 in HMV), so this is a good saving (about 20%).