Speaking of the Alfa, I dropped in on the garage at Newick so they could poke around at the power steering. Result: puzzled. There was one fault code logged - B1067: dynamic control selector on CAN/intermittent - which probably refers to the DNA switch (Alfa's "sport mode" button), but that's been working fine the last few times I've used it and while it does control the power steering one would expect it to just result in the steering staying in normal mode. A flaky battery was a possibility, so the garage did a battery test which reported 513A which is oodles of current (and most of the 600-odd amps it's rated for - the current battery is significantly up-rated compared to the original). So it's still a mystery.
I happened to mention this all to Si at SWARM and turns out he'd encountered something similar in his Skoda - on one trip he had a progressive loss of electrics as he was driving along, including power steering. In his case it turned out that the cable from the alternator had been merrily corroding and had a high enough resistance that it was no longer supplying enough current for the electrics. I'll mention this to Paul as a possibility.