What service interval are you at? Most servicing is easy to do yourself, it will save you a wad of money. I've never had any of my bikes serviced at a garage - the only thing I don't fancy doing on the Fazer is valve clearance checks which I have been quoted £150 for, which I don't mind paying.
If you are insistent on getting it done at a garage, phone around a few first. Look up the prices on ebay (filters, plugs etc) and cost up how much you would pay, add on £40/hour for labour and then you can see if the prices quoted are a rip-off or not.
Where in London are you, Chris? I used Crown Garages Kingston (in Ham) a couple of times and they were great. Dean there used to race Yam and has them in the blood! I agree with the above though, learning to do minor servicing yourself isn't hard and very rewarding. I let Dean replace my header bearing though..
