Why do people expect a Plumber to be able to fix a Central Heating Boiler? you wouldn't expect one to repair a washing machine!
To understand the Control Logic and the sensors, etc. surely you need a Boiler Technician.
I had my system converted from Gravity to pressurised via a filling loop. I have a sluggish radiator due to a partly blocked pipe at the manifold. A plumber assured me that the higher pressure circulating would move it!!!