is the cws ball valve filling when the f/e is pumping over out the vent pipe ?
is it constant or spurts out of vent ?
is it doing it on just hw / heating or both ?
try shutting the gate valve from the cws to the cylinder feed give it about 10 mins does it stop then ?
your whole system can be sludged up, pipework, radiators etc causing a restriction so the water will take the quickest route out the vent pipe.
hi-spec plumbing & heating, March 2008