Hi lee. You have to take the whole dizzy out. Once youve done that, take the cap off- obviously, and the rotor arm. Then on the bottom near where the cog goes in , there is a c clip. take that off and the whole rotor comes out. Then you can access the screws to un do the hall sensor from the side. it just slots out.
The trouble is, i can't bloody find a hall sensor anywhere! Might have to phone vw dealers and see how much they are. I'll probably just buy saul's whole dizzy. £25 sounds like a good deal.