Thanks,
looks like it might have been a knackered o-ring on the inlet side of the pump. As a test I tried blowing back through the pump (Justin at ERL's advice - thanks) and there was no flow - great. Justin had sent me a couple of 90 degree bends so I took the inlet bend off and found the 0-ring in a bit of a state - the water ran was running back because air could get in there. New bend fitted and all ok.
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