Gearbox speedo drive

Hi Guys
I've carried out a search but not found what I need, but I'm sure it will have been covered here before.
I have a electronic speedo and have had no end of bother with the pick up point. So I'm hoping to take the pick up off the gearbox drive rather than the driveshaft.
Assistance would be much appreciated.
Yes that's an important point it's a Un1. Not sure on the type of sensor but it can pick up on bolt heads.
The gap between the bolt heads on the drive shaft is not constant so intermittent reading.
It looks like the speedometer cable is held in with a spring clip .. Where can these be purchased please.
Thanks again

Ian Anderson

Lifetime Supporter
I have a UN1 in the Gt40
i used 2 magnets araldited to the outside edge of the inner cv adaptor
a pickup senses the magnets passing And sets up pulses.
the speedo converts the pulses into a reading.

i got everything from ETB