RS Tuning is in Leeds, they do up-rev tuning, although as everyone will tell you, get the mods on first then tune after.
This is what they did for my DE, although it's not really about the power it's more the throttle response and drive-ability that improves. Watch it though: when the throttle limitations in 1st and 2nd are lowered or removed you can snap the rear end out much more easily if you aren't careful,