Please can anyone help? I have a Glow-worm fuelsaver mk II boiler. Gas Central heating normally works fine. No recent changes or improvements done. Came home from work tonight and no heating. Have bled all radiators and 2 upstairs rads came on slightly for 15 minutes or so and then went cold again. The water that came out of the rads was black - could there be a blockage? Not sure what to do next - any advice would be helpful, thanks