Thanks TBM. I see you must have a different sort of heater to my '74 model. I have a bulkhead mounted box with horizontally opposed inlet/outlet pipes. One of my hoses comes from the front of the engine via the nearside side of the engine. I also have twin circular outlets facing rearwards on the blower box. Seems they added these in 1974 for demister vents (which I don't have!).
I'll look out for the valves with inbuilt bypass.
I see you have fitted your uprated air filter. I must get one to replace the grotty wire gauze filter on mine.
Is that an after market exhaust manifold?
Similar system of pipes with mine -' hot' feed comes from inlet manifold and goes into heater matrix, and then it goes from matrix to a union on the water pump. Got the valve and bowden cable from eBay - Example Bypass Valve
Made a small bracket for the heater control and cable and it fits the opposite side of the steering column to the choke cable.
Filter is a 'deep' K&N. I had a shallow one, but didn't think there was much clearance from the lid of the filter and the top of the carb so bought a deeper one.
Manifold is a larger bore stainless Librands one, and then I made a large stainless side exit exhaust to join to it.