Anyway with that in mind I bought a Brown's frame that popped-up locally. It says it's for a Fordson Major, Ford 2000, 3000, 4000 but I measured up and it seemed it would fit the Dexta.
The problem is the base units, now bolted in place, splay out, as they follow the contour of the axel. Thus at the top they are much too far apart for the top section to slot into, as the picture illustrates.

Has anyone had the same problem? I'm guessing a wedge shaped shim would correct the problem but that would probably cost a lot to have machined.
If the best solution is a purpose made Dexta roll bar, I will be appearing in the Swop forum soon.