Haven't used their plumbing service but IQ craftsmen in Crouch End have been fine for our building and electrics stuff. I can only assume that they are good at these things rather than populating their website with good content - this is very funny http://iqcraftsmen.com/default.aspx
I've used AATBF on Tollington for a couple of years now. They were recommended by guy behind the counter at Fayers plumbers merchants on Hornsey Road. Which is as good a recommendation one can hope for when it's the middle of winter and you have a knackered boiler...
As David said, they're not the cheapest - and will require payment up front for the first visit - but I'm happy enough to pass on the recommendation.
I used IQ Craftsmen a few years ago and they were awful. They were selected by my insurance company. I would never use them again - maybe I was unlucky.