Hi mate, I'm also in North London. Had a look around the area but was recommended RBM and after reading the reviews I thought it was worth the trip around the M25 to go there.
Definitely worth it, good service, good pricing and really accomodating. Would recommend taking making the journey over there.
I've been going to Ross at RBM Hampshire with my M for a couple of years including an Inspection 2 and replacement rod bearing shells and have always been impressed with the whole experience.
Luckily I'm only about 10 miles from him, but it may be worth the trip and he does have a courtesy car you can book. I know of one forum member who takes his Z4 there even though he is in Essex!
I go to RBM from Peterborough. Well worth it. I also think it will help come resale time as anyone researching a Z4 will come across RBM and will know your car has been well looked after.