just had a reread, I reckon its air locking.
That would explain why forcing mains through sorts the problem temporarily wouldn't it. But how can I get rid of the problem? I can't do the mains pressure thing on my own so end up with no bathroom until my husband comes home to help me sort it out and we are having to do it more and more often (and boy does it make an unholy mess!)
What causes airlocks?