Hi bud, yes I replaced the diode (it actually tested fine, but better safe than sorry!) and all of the earth straps are good. I have a couple of suspicions as to why it may have happened, neither of which will be repeated lol

Thanks again for the info, it's taken us a long time of tracing wires and figuring things (not to mention reading an awful lot of old threads and looking at wiring diagrams!) out before being able to actually ask the right question!