Dutton Kit Cars and their owners
I find it unlikely to be a heater, more likely to be an additional temperature sender. I assume it just goes into the water jacket not the air intake?
Its got to be some sort of thermostat for something
Its a manifold heater. Quite common in the mid to late '80s. It comes on when the engine is cold and goes off once the coolant is up to temp. Helps to improve the emissions performance from cold. Since no pinto engine left the factory in europe/uk with a DGAS I suspect its the DMTL manifold adapted to take the DGAS - not difficult to do. Unless you're looking for good cold start emissions performance just ignore it.
Sounds right - my CVH/Pierburg combo has this, but I believe not all did.