AW8, my understanding of these vehicles is that they are sensitive to even a minor fault in a single cell of the battery, which other cars may still be able to use for a while without undue hassle before the cold finally kills the battery completely. Not sure why it would drive 30 miles and only have a tantrum when it gets to the Bermuda Triangle, but I can only think that the current being drawn and then charged up via the alternator was not being maintained - ie a faulty cell not holding its charge. Interesting that the standard BMW battery fitment was only a 90aH - given all the gizmos and the time of year (have started using the heated seats on the commute to work, also the heated rear screen in the mornings - big current draw I suppose).
The 4x4 light went off as soon as I drove off; there was one more single random error which vanished and hopefully it is now happy with the new battery. I was trying to stay away from a main dealer for all-time record month so didn't want to go there unless essential!!
I do have the BMW extended warranty still in place and plan to keep it in place for the next year or two at least - car now on 54 000 miles. Probably keep it in place till 100K when it won't cover much I guess. Will need a "big" service next - 3 lights on, will need an Insp 2 this time - so hopefully check it out then.
Radio thankfully 100% restored and the only thing I needed to reset was the time, all the other settings seem to be fine.

Would have been nice to have got the battery for £85 from ECP but on the other hand, it may have done my back in for weeks hoicking it out the boot, and the convenience of having it brought to the car and sorted without any disruption to my patients was worth it. Nearly ditched the RAC after years of membership in Jan 2011 when I was marooned at the side of the road, alone, at night for over 6 hours when the S8 alternator failed and they were just hopeless - they gave me this years membership free as a result of the formal complaint I raised and I remembered they did batteries, so thought I would call. They attended within about 40 minutes even though I said it wasn't an emergency.......chap was happy to arrange recovery to anywhere of my choice if I had wanted it, but told him I was almost 100% it was the battery and please to get me a new one fitted asap, nothing else needed.
Now just to organise the wretched tail light / check brake light issue - would be very nice indeed to have an entirely 100% trouble free X for a few months - ah, to dream eh!!!!