Update 6
A bit of electrical fault finding yesterday. Thought I would test the electrics, so I connected the battery. Not a great result, immediate short circuit. Couldn’t face it yesterday, so started with a clear head this morning. My initial thought was the starter and relay, but no that was OK. Next step was to check my wiring around the DB and front cover. Something I didn’t know was that the lower mounts on the DB are insulated, the top ones go to ground. Bearing in mind I replaced the rubber mounts to solid ones. However, one of the lower mounts on the BD was missing the insulating insert, so my solid mount sent it straight to ground. That little job took about two hours. Any hoo it’s fixed now. Every day is a school day.