Try Islington Pianos in Newington Green - Steve is the tuner. I have just bought a second hand piano from them and am about to get him round for a tune. Not sure what he is like, but his knowledge of pianos is excellent and they are local and been around for years. Very nice. 020 7354 3114
A few of my piano pupils recommend a new tuner, who apparently is considerably better than the last.
I will be able to access his details when I see one of them on Sunday morning, if you like.
That'd be great. Thanks Marcus.