DIYbanter - Pressure loss on combi system

Hi all....i have a pressure loss on a combi heating system....the system is
stable when the heating is switched off....stable for over a week at 1.5
bar...but after a hour or so of activity on central heating there is a loss
.... a fairly big loss ...the trouble is is that i can't find the loss...i've
checked all radiators and all valves on those radiators...there is no sign
of any leakage to plaster work around the house....all radiators are
full...i bled them last week ...so where can it be....i've looked around the
boiler itself and have come up blank...no leaks to be found.

All help appreciated.

Simmmy