Well, as an update, and hopefully save others the time and cost of parts, after replacing the fuel cut off, rectifier and kill switch, and stripping and testing every wire and connection, I checked the carb again. Although I had taken it apart and cleaned, then blown it through with the airline, I had missed a tiny grass seed which was stuck in the main jet!! After poking it out with a strand from a wire brush, she fired up and revs a treat!! Just shows you that even after 50 years working on motors, even an old fart like me can still make "rookie" mistakes!!🙃DOH!!
Thanks again to everyone for their help👍, ps, all the answers deserve a "tick" as being most helpful.