how is this removed from the distributor? removed the 3 screws but the plate wont come over the two teeth on the shaft?

(4x4 saff cosworth btw)

 

At the bottom of the phase sensor is a magnet which sits on the bottom of the dizzy. Disconnect the electrical connection undo the screws and it should just pull up.

 

THREE screws ?
You only need remove two that hold the phase sensor to the bottom plate, as you say the bottom plate will NOT come up over the shaft, the sensor sits on it's own half circle plate on top of the main plate.
Dont forget when renewing to get the top most RED wire to the terminal in the through/connector that is nearest to it, also check carefully that is is 100% parralell to the lobe as you may have to GENTLY bend the sensor over till it is, then gap to 0.2mm - 0.3mm.
tabetha