Got the CH off (so diverter valve supplying hot water cylinder only), the thermostat strapped to the cylinder set at 65 degrees - this works fine as I've moved it up/down and system fires up/shuts down as expected. Boiler is set to level 1 of 5, pump on level 1 of 3. Water dribbles out of the hot water cylinder vent pipe a couple of times every day - not very much water, but it causes the overflow from the cold water tank (in loft) to come into play for a few mins every time. Anyone have an idea what might cause this? Thanks.