Ok got it working properly! I've left the coil tap out for now. I think the problem was my earthing arrangement, I must have been creating a short somehow, but I don't really know where it was. I kept tracing through the diagram and everything looked right.

I pulled everything apart and soldered eyelets to the end of all the earths and used a screw and some copper tape to make a really good common earth point. I rearranged the switch back to Weirdbits diagram and it all seems right now. The only real change is that I have the earth from the tone pot going to the common point before going to the jack.

Its always the way the first soldiers are the neatest - so these are a bit blobby after being done and redone a few times, but this is all good now. I'll button it all up and spend a bit of time doing setup and then I can always come back and do the coil tap and tidy it up a bit later.