I posted a few days ago about a water leak on a Worcester Highflow 400 boiler(posted on the 20th of Nov).
Anyway the cause was due as the Worcester engineer assumed caused by a blockage on a braided hose that went to the expansion vessel. Looked more like the hose had collapsed internally to me. But he reckoned that the system needs cleaning of some sludge and suggested a power flush (probably to cover themselves). I've got a rough idea of the price of a powerflush about £250-£500, very expensive. Would it be just as good if I put Fenox super concentrate restorer in the system for a week and then flushed the system through using the mains pressure from the filling loop and selectively opening the valves on each radiator in turn. The system has the boiler upstairs with the circuit running around the upstairs of the house pipes dropping down in turn to each room downstairs. Each dropdown has a drain valve fitted.
Since the boiler was fitted 3 1/2 years ago 5 out of the 10 radiators originally fitted have been upgraded to double panel radiators and the water looked clean at the time so where black sludge has come from I don't know.
cheers Julian