Thank you Mike, I'm glad someone was paying attention!
Some clarification is in order I think &, Mike, please correct any errors I might make in my description of how I believe a gravity heating system works.
When the the water in the back boiler of a solid fuel stove heats up, convection will occur in that the heated water will rise to the top outlet, drawing cooler water into the bottom inlet of the boiler. This heated water progresses through the top of each radiator as the cool water at the bottom of the radiator is drawn out towards the boiler & so you get a circulation without the need for a pump. Any calorifier installed in a gravity system is, in effect, just another radiator that heats water rather than air space, provided it has a suitable coil.
Now, my situation is that because I can't totally isolate the calorifier from the gravity system like I can with individual radiators, any heat contained within it is being transferred to the cooler water within the calorifier's gravity coil & despite the fact it can't circulate because the bottom gravity valve is closed, that heat is being dissipated some 6 or 8 feet along the top "hot" pipe of the gravity system to either side of where the calorifier is tee'd into it. Thus, I'm losing all my hot water overnight.
My thinking is that because the calorifier is installed in exactly the same way as a radiator in my gravity system, fitting an isolation valve in the top "hot" outlet of the gravity coil, should prevent the heat being dissipated to the rest of the gravity system, or at least slow it right down to the bare minimum. All I was asking is whether my thinking is correct or whether I should consider an alternative solution. The problem is totally unrelated to the Webasto or the engine/generator as these are not capable of heating the calorifier at the moment, they were merely mentioned to give a complete picture of my installation & maybe I shouldn't have mentioned them at all.