A bit of a breakthrough:
I noticed that the trip odometer has been changing very gradually.
So I wondered whether, considering the roughly 50% under reading at present, had I actually been going fast enough for the speedo to register?
So I went a bit faster.....
And the Speedo started to work!

So it's a calibration issue - of course speedos often don't register at very low speeds (the highest I'd reached as an indicated 15km/h - which is around 10mph)

So nothing fundamental then - phew!
My Sierra VSS is either faulty or just behaves differently different from a Focus one.

What I need to know now is understand the difference between a Sierra VSS and one from a Focus in terms of pulses per mile/km - whatever - and do the necessary conversion