We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Keefiedee
saying something like:

The only fail-safe answer I can come up with is using 4 "normally
open" motorised valves, wired so they close when the boiler comes on,
but these cost a minimum of £65 each for 28mm valves as far as I can
see. So I was toying with the idea of just fitting two on the central
heating side of the woodburner pipework and somehow working out if I
could make do with non-return valves on the HW side - if such things
existed - hence my question.

You don't need two of anything. Just the one would do, but as a general
rule, nothing that could stick closed should be put into a gravity loop.
It works, that I do know - I've come across it, but I wouldn't do it,
just in case.
Have you seen the results of a steam explosion in a house?