Can a heating engineer offer some advice, please?
this sounds like a right mish mash,normally a single pipe system is gravity fed,from the fact that you say there are 2 m/valves fitted would point to it being a fully pumped system,the rad in the bathroom is fed from the primaries(the main flow and return from and to the boiler),the idea being that in the summer when you’ve got the central heating turned off and hot water selected then the rad in the bathroom will heat up. i used to be a heating and chemical engineer working on problems like yours everyday, i think you could have a partially blocked cold feed, the black muck you get in radiators is called magnetite,it’s caused by a combination of oxygen in the water and the reactions of the different metals within the system,the blockage in a cold feed is a different consistency and is mostly like a hard light brown clay,this can be caused by pump-over,if you manage to get someone back try and convince them that they need to cut the cold feed out where it tees into the system,normally
