it sounds as though the plate heat exchanger is cross leaking and putting mains water pressure into the heating side.
If pressure is rising when boiler isn't running and filling loop key is removed then this is more than likely the problem, hope it helps.
joe, October 2008
thanks-filling loop valve(gery knob)offand key removed.
shall i try c/h on mains water off and see what happens to preasure then?
shaun, October 2008
Your filling loop must be still letting water through disconnect or you may have valve to turn off
