I got round to pulling the rubber grommet off the chassis plug today, and found that the thick red wire has broken, but as far as I could tell it was the only one that had severed through and through (see the first photo). I am guessing that prior to me removing the grommet it was only touching sometimes and hence causing the intermittent fault. At this point none of the functions to the door worked, I surmise the thick red wire is probably a common 12v +ve feed.
I thought it was probably worth having a go at a fix today. I managed to strip back just enough of the red insulation to allow a self-soldering joint to slide over both ends of the break, inserted a thermal shield (i.e. piece of cardboard) between the red wire and the rest of the bundle to prevent collateral damage, and very carefully melted the solder with a 175w iron. The joint had its own shrink wrap, so that should give some mechanical protection (see second photo):
The break had been close to the door end of the wire, so I couldn’t get the soldering iron close to that end.
Now I seem to have full functionality of the door: central locking, mirrors, windows and speakers, so this appears to be a good quick win for the time being. The wires are quite brittle, so this probably isn’t the end of the story. I have ordered the repair patch, so I think I’ll still take the door apart and fit it next weekend. I’ll report again when I do that job.
Many thanks for all the advice chaps,
Alan