I replaced my hall sensor on my '93 Cav Turbo last weekend so here's how,
Remove distributor cap using female torque and move out of the way.
Remove 2x 13mm bolts that hold the distributor to the head.
Remove distributor.
Remove the rotor arm and plastic plate.
Carefully withdraw the clip that holds the black plug to the distributor (this is on the inside).
Unclip the black outer plug from the hall sensor plug.
Turn the dizzy the other way up and using a punch (I used a nail) tap out the pin that goes through the shaft.
Next remove the shafts from the top and bottom. Watch for the washers falling out.
At this point note which way round everything goes and the order of the washers - you can put this the wrong way round and it'll be 180 degrees out!
Remove the screws holding the hall sensor in and withdraw from the distributor.
There are some pics on a thread on MIG here (
http://www.migweb.co.uk/forums/showthread.php?s=&postid=692624#post692624)
I've seen threads that state that a new hall sensor is about £15 but it was closer to £35-£40 when I got one. Its a bosch part so you should be able to pick one up from a bosch supplier or Autovaux can get them.
The HES part number is - 1237 031 288.