I looked at these before coming back to dukes (even test rode a 1200 for a few hours). Sub £3k is not a good idea, and even £4k doesn't get you a good'un. GS'er UK is a great club though:
http://ukgser.com/home/ I'd go for a decent 1150 for about £5k. Moto Uno in Oz seem to reckon the engine's every bit as good as the new one, with more torque and nearly the same power (not that bmw see it that way...)
Ali