CHRISWAG said:
Not sure of pump type but definitely running on both programmes
A common mistake with circulating pumps is to make as assumption that they're running when the only external indication is that it's vibrating.
You need to check that the spindle is turning - remove the end cap (with a bowl or an old towel underneath) and check with the end of a screwdriver that it's turning.
Even if it is, the pump might still be ineffective - the impeller could be worn, broken, or loose.
I'm still curious to know whether the system is sealed or open-vented - do you have a feed & expansion cistern (aka header tank) in the loft?
If so, take a good look inside it and assess the quality of the water. Scummy and dark is ok, but rusty and/or sludgy is not. Also, note whether any water is dribbling or pouring from the vent pipe (over the F&E cistern).
Two two port motorised valves
A failure mode that can case boiler noises is for both valves to be shut with no system bypass. A picture of your pipework around the cylinder would reveal whether or not you have a bypass there. Although MZVs are intended that the boiler should light (and pump run) only when they're open, I don't know whether or not your system is correctly wired.
The builder added a new drain off valve but when emptying the system did not drain off the old existing drain off valve. He did not flush the system and did not add an inhibitor
This is a good indication that he wasn't thorough. I would never add a radiator to a system and not drain it unless I checked the state of the water, i.e. degree of sludge and concentration of inhibitor - did he do that?
By the way the system is a single pipe system which I know is old hat but it has worked perfectly prior to this work being carried out.
Try not to be misled by the fact that the fault is sudden - all components fail in time, so it's commonplace for things to be working perfectly right up until the moment that the fault occurs.