Speedo not working

1981 Series III
Speedo isn't working.
I've put a drill on the end of the speedo cable and the speedo registers.
Fitted a new cable but still no joy
Left new cable fitted and disconnected at the speedo end to see if the shaft in the cable turned when driving along - nope, not turning
I'm assuming its the bit in gearbox that drives the cable.
Can this be replaced without taking the gearbox apart?
If so, what's the part I need (part.no. / description)?

remove 2 x screw on speedo housing and pull out ,when refitting watch out for cutaway on housing it has to go in output housing right way round so it doesnt foul speedo worm