There is a source here in the States. It's called Sports Car World. More than likely they have the heater box. There web site is virtually none existent. They will be expensive, but will have the part. However I would give Richard Winter a ring first.
Here is the information:
PH.(254)471-5539,Fax(254)471-5617
253 LANGFORD COVE ROAD
EVANT , TEXAS 76525 USA
9 to 5 MONDAY TO FRIDAY CENTRAL TIME
WORLD WIDE SERVICE
We Accept Visa , Mastercard,American Express,Discover
IF YOU HAVE A LOTUS YOU HAVE
FOUND THE SOURCE