I have a worcester Bosch 28i junior combi boiler and recently it has stopped supplying hot water for the heating. It fires up ok for the hotwater and supplies plenty with no problem,but when I try to put the heating on it tries to fire up and does only briefly. It still appears to heat the water upready to send around the radiators but it only goes approximately 6 inches into the pipework and doesnt seem to be pumped any further? I have drained the system down and bleed the radiators etc but with no success. I have also checked the water pump and central heating controls which are both fine. Please help?