Ok, I’ve just had a chappy around, he’s checked up and down, had his camera out and checked all around and agrees that there must be a whole load of debris in the system, he has suggested a pre treatment to soften up all the gunk, disturb the rads, fit a filter to the system, clean it all out, but he’s also suggested that the air separator(?) be removed as they can bring their own problems, and also something I didn’t even know was a thing, but to remove the gravity side of the system by fitting a pressure reservoir, which should eliminate so much of the rusty sludge side of things because the system wouldn’t be vented anymore. Mind blown.