Thanks Andy. One of your points has me thinking. This 'sucking down'
thing. I once cracked the upstairs bleed valves with the pump running
(bad practice I know). The needle valves sucked in air rather than
released water. Is this syptomatic of the problem you quoted or would
you expect that to happen every time? And if it is a sucking down
problem how can it be remedied / prevented?

Al.