Top boiler up to 2 bar isolate boiler and leave over night
If boiler has not drop pressure not the boiler.
But when you open isolating valves on boiler and pressure drops you know it's pipe work or rads
Make sure you turn off at fuse spur
What boiler do you have, if it's a combi it could be scaled up plate exchanger, when you run the shower does the boiler fire up and then go off and fire up again
Paul
Why not call another engineer, you have replaced stat and pcb which any engineer would have told you thats not the fault.
You are out of your depth so get a pro in it will save you money esp if your going to chuck parts at it
Paul
You could fit an auto air vent on rad, your need to check why system is drawing in air.
What pump is set at, is vent pipe correct height , where is the pump on the system
Paul
installed the same toilet in my house, had no problems looks ok.
Problem i had it took 4 toilets before i got a good one
1 had a crack where the soil pipe went onto
2 was in bits
3 toilet cracked round rim
Paul