I finally bit the bullet & stripped out the transmission tunnel top & side panels (many many M4 bolts) & took off the speedo sensor. This allowed me to run the entire speedo system on the bench by twiddling the square shaft on the sensor (rather than having to drive the car). Guess what? Nothing. Speedo didn’t move. So was the problem the sender all along? I started the engine & swapped the speedo onto the tacho plug & it worked, so it could ONLY be the sender (fingers crossed).
So I have now purchased a Fiesta MkV speedo sensor & a Lambda port plug to modify to fit the fiesta sensor to the gearbox – I did this once before on the Rickman to get a speed signal into the Ford ECU. PLEASE may it work this time.