Boiler replacement scheme - safety

I agree with you, but the plumber wasn't going to fit those controls, hence this thread.

Yes, when I installed a system in my previous house in the late 1960's, I used a (Drayton?) Cyltrol in the HW circuit on the output side of the cylinder. That worked in a similar way to a TRV, cutting off the flow when the return temperature reached a certain level. It didn't provide boiler interlock, of course, in the way that a C-Plan system would. It worked fairly well most of the time - but just occasionally the gravity circulation decided to go the wrong way round, whereupon the output became the input - and the bloody thing closed down with the tank still cold!

Dunno how good they are, though. I

I doubt that anyone is installing gravity systems today - but I would guess that there are still a lot around that have never been converted.