I suspect it's a combination of very little driving, the recent cold snap, a 6 year old battery... and putting a decent amount of air into the spare tyre last weekend (I've probably not checked it since last year).
Fortunately a neighbour had a set of jumper leads, and even more fortunately this is the neighbour who's parked next to my car! Wiring up the leads was a bit of a challenge as the engine bay of a modern car is basically all plastic and we struggled to find a chunk of bare metal that was in reach of the negative lead and the crocodile clips would actually hold onto... but we found a bracket on the side of the engine, my neighbour gave his car a bit of revs, and mine reluctantly decided that yes, it would actually start the engine.
Once mine actually started it ran nicely, so I carried on to Fareham sainsbury's via Emsworth to put a bit of juice back into the battery - which seems to have worked, as when heading back home afterwards it started happily (my backup plan if I was stuck there was to call the AA).
Driving along the M27 in low gear (to recharge the battery) is a bit disconcerting with the extra revs and power. I normally climb the hill past Portsmouth in 6th gear, but drop a gear or two and the extra power means MiTo will happily keep going past 70mph if I let it!