Potterton Promax 24/2 PROBLEM - HOT WATER OK BUT NO HEATING?

We have had this boiler for about 2 yrs. I have the heating on constant with the thermostat controilling temperature. When the snow arrived in Dec 09 we would wake to a freezing house each morning but switching boiler off & on again at the wall would sort this. Since the weather has been milder the boilER started to behave again but now we have no heat but the boiler is working for hotwater.

Took the case off the boiler and there is a black casing that looks corroded on the side of the main part of the boiler. that has been dripping down onto wires inside the casing & looks wet. Any ideas what this might be ?? Surely the inside shouldnt be wet??