The problem is now resolved! As suggested, I managed to completely dismantle the bakelite block and it's various components. It's not that straightforward as the end of the fixing that protrudes on the outside of the front, is piened outward to hold the felt washer in place. The other end of this is a hex head, held in place with a tab washer. I had to remove all of the felt washer and any of the remaining bent over bits. I also had to cut a slot in the hex head to get more purchase. The part is smooth on the outside and threaded internally to take the screw at the front. The hole in the casing has an insulating tube, which together with the felt washer stops any short circuit from the brass tab through the body....... well in theory anyway! I had to remove the remnants of the end that had held the felt washer and in doing so, lost most of the protrusion beyond the front of the casing. I also had to straighten the end and re-tap the internal thread. With all the parts out, it was easy to re-solder the fixing post for the points to the brass plate. I used a sticky felt pad (used for furniture legs) with a hole punched in it and an insulating washer, brass washer and spring washer, so I could tighten the whole thing and still keep it insulated from the body. I re-tested for short circuits and all now ok. The cherry on the cake was putting the whole mag back together with the original coil and getting a spark from casing to coil via a bit of brass wire when I spun the rotor.
