[Mgs] MGB Heater not working

[Bert Palte]

Well fellow listers, I haven't seen any messages here for a long time, 
so here we go.

I rebuilt my heater some time ago because of a leaking core.
Not a very easy job, BTW.

The heater has been functioning correctly for some time, but now I can't 
seem to get it working any more.
What could be the problem?

Apart from the core, I also changed the heater valve and the fan. The 
fan is still working.

The car is a 1970 B Roadster.

TIA

Bert