Finsbury Cycles. Is that close enough for you? Otherwise it's Stoke Newington, Tottenham or Crouch End. Any that have opened up in Wood Green or Green Lanes have ended up closing down.
There's a very good bike mechanic who has a repair hut just by the entrance to the Castle Climbing Centre on Green Lanes - this is his website http://www.bikemech.co.uk/
good and cheaper than most shops. I have to say learning to fix a flat tyre is one of the more useful things in life to learn!