I have the same problem. You have to replace the inside part of the tap - the plumbers' merchant said they get grit inbetween the plates that turn the tap on and off; once those are scratched they always leak. They're trying to get me the parts (Diamond, Acre Lane Brixton). To get into the tap, lever off the central cover with a tiny screwdriver, then unscrew crosshead screw and undo main bolt - you need an 18mm flat spanner or adjustable one.
LH London, April 2011
