Geoff , if you suspect the boiler is losing pressure carry out the following
Switch the power to the boiler off, pressurise it to 1.5 bar and isolate the flow and return (the service valves are usually beneath).
Leave overnight and note the pressure. A drop will indicate the problem is within the boiler otherwise it must be in the CH circuit. This assumes that the PRV is not letting by.
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December 2009