Hi, and welcome!
I think you will get as many different answers as there are people with 749's on here.
Some, like mark749s got a 6k mile/6 month intermediate service. I just changed my oil/filter myself half way through the year, but then my mileage profile is different, about 6k miles in a year.
1st service is 600 miles, and some dealers offer this as cost of parts only.
Next is 6k miles or 1 year and costs around £400/£500, but can vary a lot depending on what needs doing (like how many valve clearances need adjusting)
2nd service is 12k miles or 2 years and is the same as above but also includes changing the belts (which cost about £100) and the fork oil. Expect to pay around £600, but could be a bit more (same as above, valve clearances)
There was a thread active a couple of weeks ago where a lot of costs were quoted.
Sorry, no idea about servicing in london, but I have heard a lot about ProTwins.
[Edited on 27-11-2005 by HW]