combi boiler pressure loss?

baxi 80e combi losing pressure. Only used to lose pressure with heating on. Used to have to top up every couple of weeks, now every 2 days, 1.5 bars to 0.3 in 2 days.
No visible leaks upstairs so had plumber round to check below floorboards and found no leak.
Topped up to 1.5 bar shut off flow and return to rads.
Pressure dropped to 1.1 over a few days of using hot taps then has remained at 1.1 for several days.
Just turned flow and return back on and the pressure remained at 1.1.
At a loss to work out whats going on.
No discharge out of over flow (put condom over it).
Boiler serviced a few months ago engineer said to try fernox leak sealer but no joy.

gameoverman
April 2009