I just wanted to add:
when you go look at the bike, don't let anyone pressure you into buying it if you are not 100% sure on it, many times dealers or friends will try to push you to buy it. keep a level head, mark down anything about the bike you're not happy with and ask as many questions as you can.
when they know you're in two minds, they usually say "you can secure the bike for a refundable £100 deposit". this is entirely up to you, but if the bike has been there 3 weeks, and you're still not sure, I doubt its going to sell in the next 2 days. if you haven't left a deposit your mind is still open to other bikes, where as if you leave a deposit you kinda will always be thinking about it.