So after working through a few bits and pieces on the wiring, the final answer was much easier than anticipated. It was quite simply a fuse !
It would appear, for reasons not 100% clear to me just yet (and actually not sure I can be bothered to work it all through) that the same fuse supplying the interior light and radio (I don’t have one fitted) also powers in some way the diagnostic port/circuits for the engine, abs and airbag ECU, but not the Pektron SCU.
The fuse was blown (probably because of the radio wires left dangling). So one new fuse and a tidy up of the radio wiring and it all worked.