Fixation UK
The best Nikon Service centre I have ever used!
http://fixationuk.com/Fixation/Repairs.html As used by top pro's, photojournalists, and loads of agencies in London.
You only have to look at the size of their repair team to see how in demand they always are, I can't recommend them enough, they normally do my sensor cleans while I wait and have a coffee, and they are very quick and efficient with repairs.
Oh and with regards to getting a pro to clean your camera, for me its not about whether they do a better job (although 9 times out of ten they usually will), but more about the fact that they are insured if they damage your sensor whilst cleaning, whereas if you do you're fucked!
The question I ask myself is, who do I trust more to service my camera body without causing a repair bill in the region of several hundred pounds....
me or the people that do it for a living?