Millenium are very good, but there's also a very good garage on Abbey Street called General Autos (020 7237 3444), just by the railway arch. I've used them a few times now and they're very good. They're also not in the CC zone.
I have no connection with them, save that I've used their services on a few occasions.
I had a complete engine rebuild on my black cab at Blackfen Garage, pistons, liners, the whole nine yards, it was known as Blackfen Taxis to me at the time.
It was off the road for 7 working days as the parts had to be ordered and delivered.
As I recall the bill was around £2150.00 which I considered to be a result at the time. The cab ran like a dream from then on until I finally sold it some 7 years later.
The proprioter's name was Norman I think, and he was a perfectionist.