Hi there, I am no expert BUT??? I have just had the same problem myself and sorted it out myself at very little cost.
First you have release the pressure by either useing your PRESSURE SAFETY VALVE it is red in colour with a black pull tab (number 27) in your instruction book (page 55). this will release the pressure which the water will then go outside via your overflow pipe.or, draning water from your radiator
Close your valve that fills the loop the one that you have to use to build your pressure up (number 7 in your instructions) make sure it is in the horizontal position. remove one end of the loop ( No 8 flexible hose ) make sure that you have a container to catch any excess water.
Turn your valve back to the upright position, NOW !! if you have any water coming from your loop then your problem is your STOP/FILL VALVE (No 7) which is allowing small amounts of water though which could be the problem of your pressure build up.
makzy
November 2010