Anchor Supplies got back to me and they haven't had any for a few years (but which means that since I've been going there at least 20 that at some point I've wandered past them, thumb up bum, mind in neutral) but they have put me on a waiting list in case any do turn up.
The guy didn't patent the design so the army could make them without hassle but there is a patent for a 1959 conversion to gas or liquid fuel
GB0819804
FLAVEL & CO. Ltd., S. Jan. 21, 1957 [Jan. 20, 1956], No. 2067/56. Class 126 A conversion unit enabling a Soyer field kitchen 1, Fig. 1, to be heated by a gas or oil burner ...
But i can't find a digital copy of it!
I'll keep poking about
ATB
Tom