My heater control is stuck on hot, making driving in this weather extra uncomfortable in the cabin.

Can I close off the hot supply to the heater in the engine bay, with a hose clamp or something, without interfering with the engine cooling  circuit and causing damage to the engine.

Can someone please confirm this is the larger of the 2 pipes circled in the pic, coming out of the cylinder head close to the bulkhead.

Yep that's the heater hose. Some have proposed that the heater hose promotes coolant circulation in the head #4 rear area, as the other exit is the elbow between intake runners #3&4.

I would assume for an early car that would only apply when the heater is on, and I don't know if there's any history of #4 running hot. You could use a wide hose clamp on the hose if it's not too brittle and see how it goes.

Clamping the hose should work, but you'll have to be careful to get it fully closed without damaging the hose.

You could also fit a bypass hose, from the heater take off in the picture directly to the heater return at the thermostat housing.

Just found the problem of the heater control being stuck on hot was because the cable/rod was stuck in the plastic cover by a congealed substance, probably very old grease. Stripped it out and cleaned the cable and cover and it's working great.