Speed sensor signal problem

Getting really fed up with this. Did a box swap about 8 weeks ago, since then the speedo hasn't worked.

Problem:
Gearbox taken out of car has a separate plug in speed sensor.
Gearbox put in car is a later version with speed sensor integral in the box. The wiring loom is different on the new gearbox.
From this site I got wiring diagrams that showed both gearbox set-ups and it seemed a pretty simple job to take the signal wire from the 'new' gearbox and link to the loom on the car that had the plug in sensor.

We have a pulse from the gearbox sensor, but when connected to the car wiring loom, the speedo still isn't driving!

Are there differences between earlier and later speedo heads or speed sensors?

HEEEELLLLLPPPPPPPPPPPP!