Ok - car is now back from the auto-electrician.

He told me that he had "quite a job" (don't they all?), and that when the car was linked up to his rig, it was reporting no draw at all when the car was off and locked up. He claimed that this was the 7th car in a row that had done - sorry, if I come across as unnecessarily skeptical!

Anywho... after having had the car in his garage for three days, he reckons that he's traced an intermittent fault back to the amplifier in the boot. He reckons that this has become water-damaged at some point (which, to be fair it has), and that the plug was showing corrosion, and that water had ingressed onto the amp circuit board. He then reckons that the amp was sparking into life intermittently and regularly, and bringing other systems on that are on the same fibre optic network - i.e. the rest of the MMI system, the DVD player in the boot and so on. He tells me that as soon as the amp was disconnected, all of this stopped immediately and didn't repeat itself once.

Therefore, he has left the car amp disconnected and given it back to me. He's told me to get a replacement amp from Fleabay, and that once that's arrived, to pop back, he'll pop it in FOC and check that the systems are running normally after the new one's in. I picked it up Wed eve and drove it home. It's been on the driveway since then and I'll be using it later today this afternoon - will see if there's any signs at all of "low battery" warnings on the dash or any potential issues with starting it.

As I've reported on here in the past, I had the dreaded rear wash-wipe pipe break, and had all of the internal casing out to replace the pipe to rectify. I actually found this fault as I was buying the car, and testing the rear wiper and trying to work out why no water was coming out. The amp/DVD bay was drenched as the water poured into that cavity, but we clearly hadn't a sufficiently solid job of mopping up all of the water.

****. £216 to an electrician to root the cause back to something I already knew about. I knew that the MMI system was the prime candidate for being the root cause of this issue!

New amp is en route. Hopefully, this is indeed the root cause. Should know by Mon/Tue next week...