[Mr Teddy Bear]

Beef on this forum was marketing a aluminium alloy housing modification to replace the plastic one. I have one of his leaflets some where.

[crepello]

The longer the thermostat is closed, the quicker the water in the engine will reach operating temp, as it doesn't have to overcome the cooling effect of the extra water in the heater matrix and system and any heat being distributed into the cabin on those very cold mornings.

Not entirely. I'm spending quite a lot of time thinking about mine at the moment, as a 'slow and steady' alternative to ripping the head gasket out again!
When you sketch out the K series water circuit, it reduces to three parallel loops external to the engine.
The 'large bore' loop goes through the radiator, returns via the coolant rail to the front of the thermostat.
The 'major small bore loop' skips the radiator, goes through a jacket in the inlet manifold on Carb and SPi engines, or through a junction block on MPi engines, and returns to the back of the thermostat housing.
The 'minor small bore loop' branches off the 'major small bore loop', runs to the heater gate valve, through the heater matrix, then back to rejoin the 'major loop' between that exiting the inlet manifold and joining the back of the thermostat housing.
The 'major small bore loop' is the one loop free of valves, and presumably it was a matter of careful design to make the relative loop flow resistances balance such that flow through the radiator and heater matrix are sufficient when their valves are opened.