A friend of mine found a nice Smiths heating unit with the optional de-misting attachment for me which I was very grateful for. I had to modify the manifold heating pipes and engineer a flow tap for the heater out of an old ball valve, some steel tube and a brass wingnut that I had laying around in the shed. I used an already existing hole in the firewall to mount it all to as it is sacrilege to drill fresh holes.

The Bakelite pull switch on the dash activates the heater, and the cabin can be warmed by opening the front flaps when de-misting is not required.