Quote: the system pretty much keeps stable whilst powered on and hot water is running but when switched of it drops 1.5 bar every few hours
Did they check the expansion vessel?
Try this...
Allow system to cool
Repressure to normal setting
Turn on heating and watch what happens to the pressure.
If the pressure rises up to say 3bar and water comes out of the pressure relief valve then the expansion vessel is likely to be the problem. Could be faulty OR simply too small now that the system is bigger after the garage conversion increased the volume.
Edit: The fact that the pressure falls when the system goes off is also a clue that this is the problem. I think it's unlikely t be a leak if the pressure is stable while the heating is on.