I've never had a good experience at Evans London Bridge. I usually find that I'm ignored for about 10 minutes or until I lose patience and walk out...
On the other hand, the guys at 'On your Bike' on Tooley Street are really friendly and helpful. I popped in on Saturday afternoon with a flat tire and they let me change it in the shop! :o) I sevice my own bike so I can't comment on the services but they're a local shop (i.e. not a chain like Evans) so I think they should be supported! ;o)
Oh and if you haven't had your bike serviced in a while, you can expect your bicycle chain to need replacing.
If you have a mountain bike or something similar then you might want to ask them if you need new front or rear chainsets as the teeth wear down after a while, causing the chain to 'jump' (which you really don't want).
I would avoid Evans cycles at London Bridge. They ruined my gear system and then accused me of tampering with it when i took it back. All I did was take it in for a service. Everythnig was working well prio to their handywork. Cant speak for any of thier other outlets.