loss of pressure

I have a combi boiler which has been losing pressure.

Pressure would drop drastically overnight and I was told that there must be a leak in pipes buried in the concrete floors.

Specialist contractors with thermal imaging were called and they used a specialist gas to highlight any leaks. They checked the whole house and only one area showed up.

This was then dug up but no leak was found. Further investigations followed but mysteriously the pressure stopped falling.

This was stable for several months but has now started falling again although more slowly.

Only two things have changed - I bled a radiator and also switched on a kick space heater that I have not used before.

Once I bled the radiator (when system was cold) I topped up the system but pressure falls. Could this be the problem and will it settle down?

I assume all the usual checks have been carried out on the boiler, inluding PRV dripping, Expansion Vessel Failed or needing recharging with air, Expansion pipe blocked? etc, what make & model Boiler is it, how old is the Heating system?

thanks for the reply.

Yes these were all done previously and prv is ok. We also had a second expansion vessel fitted outside the boiler.

It is a ferroli and only a few years old

How often do you have to top the pressure up, and how much do you have to put in?
It could be mild form of this.

Which Ferroli ???
Condensing or not???

Main heat ex's leak at the back left.... noticed any water inside the casing??
2ndary h/ex could be leaking internally as well if its a steamer

thanks for the replies.

It is a ferroli optimax condensing combi boiler.

Have not seen any water in the casing.