Update... After setting the timing correctly I took the dizzy cap off and marked the position of the rotor. I lifted the dizzy out and rotated the whole thing anti clockwise and reinserted, then rotated it to line up my mark with the rotor. It either goes back to the same position (where the dizzy is rotated past the bolt hole) or too far the other way.

I then tried removing the dizzy, moving the the rotor round one tooth and reinserting with the same result.

After over 2 hours I have set is as close as I can, but the timing is advanced and it's looking like it's not going to be possible to set it correctly.

My only option is to do what another member has done and file out the bolt hole to allow me to reach the correct timing. There must be something else causing this that I am missing...?