I have been called in to sort a friend's heating system. They were constantly bleeding some of the radiators! The plumbing is a bit of a nightmare, done by the owner but cut and changed over the years. Originally there was a backboiler heating the system, this was removed and an oil boiler added outside. I moved the tank in the loft to a higher position, didn't seem to make it better. I decided to flush the system with a desludger. I opened the drain at the boiler and got about 30seconds of black then clear water! Then nothing. So I opened the cold supply to boiler from rads, an initial flow for 30secs then nothing! What can I do now?