It is most likely the diaphragm in the diverter valve as suggested.
I was quoted £silly to replace mine, so I bought a diverter valve and the washer kit (o-rings and fibre washers) and did it myself.
Took around 1/2 a day, but it's quite simple, just remember to turn all the taps off on the pipes at the back of the boiler and drain the boiler. Otherwise, it soaks your kitchen when the guage sensor is taken out :S
Wayne, January 2009
