Have hot water but no heating?

We've got a potterton suprema boiler and a separate hot water tank. The boiler still fires up to heat the water but have no heating. We've tried replacing the myson valve which doesn't rectify the problem. We can manually move the leaving in the valve housing to have the heating on but its either on or off. Could this be an electrical problem as the valve can be turned manually and didn't work with a new one. HELP!