Problems with my one pipe system, I´m desperate

I have got a one pipe system in an old terraced house I recently bought, I've had a plumber there, take out the old primatic cylinder and replaced the cylinder with an indirect cylinder and new header tank. The back boiler is working perfect I've been told. We've replaced the pump and drained, cleaned and flushed the system. We've bled the radiators but the system isn't working, the boiler only fires up when you are bleeding the system, what's wrong?, it´s got the plumber baffled.

boiler

Without being there to see and test the system we may not have a good answer for you.

When you say "one pipe system," do you mean a steam system or a monoflow forced hot water system? You mention a pump which suggests forced hot water. I always check the thermostat before I go to crazy with anything else.