Just in case anyone ever has the same problem, I’ve now sorted it. The green (signal) wire doesn’t connect to the coil for a signal but instead goes to the rectifier under the seat. Once it was reconnected the tacho works fine, although judging by the amount of grime on the terminal it had been off some time, which is why I missed it when I first looked.
[IMG][URL]http://i198.photobucket.com/albums/aa249/NickAtk/th_MZ%20Rectifier_zpsjdvn1i2t.jpg[/IMG][/URL][/IMG]
