a cheap digital voltmeter which is totally sufficiant for this will cost you from 5-15GBP. They come even cheaper on ebay but no matter what the sellers location says most are shipped from China and will take weeks to arrive if they will at all. I`m sure you`ll find one locally or know somebody in the neighbourhood who has one you can borrow.

I feel a trip to my local parts supplier for a voltmeter coming on, together with a steep learning curve. I'm trying to avoid paying out at the main dealer again if I can help it.

you don't need anything fancy to measure what is needed. Google voltage drop testing. Easy to do and will pinpoint any wiring issues.

so ... ?