My Potterton has started to loose pressure when the central heating is switched on (hot water appears fine) with the needle dropping into the red and the low pressure light coming on. If I re-pressure the system it starts working again Ok but there is water dripping out of the bottom of the boiler and also the overflow valve on to the patio. The pressure then continues to drop until the boiler cuts out again when the low pressure light comes on after aprox 4 hours usage. Any ideas - is this something simple or big and expensive ? Boiler is 10 years old - bit worried about the dripping water causing some electrical fault.