+1 what embee says. Paperwork is very important. Have a good look round it and check all the obvious things for signs of wear or damage: chain, tyres, brake pads, wheel rims, etc, and scratches on bar ends, footpegs or casings that may indicate a drop. For peace of mind cough up for a HPI check if your parting with a big wodge of cash.
Have a look at these buying guides for a bit of info about what to look for
http://www.autotrader.co.uk/BIKES/bu...op_10_tips.jsp
http://www.autotrader.co.uk/BIKES/bu...ing_a_bike.jsp