Then I got to confuse everyone in the queue of traffic by stopping, switching the ignition off, restarting the engine and then heading merrily onwards with now-working power steering.
I suspect battery - this was just after starting, on a tight turn (on this particular roundabout I do a U-turn and exit the same junction I enter on), and additionally at the weekend I'd been running the tyre compressor off the battery. Also the only thing that failed was the power steering and not all the electronic stability gizmos. Given that the Alfa still has the original rather wimpy 6-year-old battery it's not too surprising that it'd be time to replace it.
Besides, the other common failure is the torque sensor in the steering column packing up and that's expensive to fix. Batteries are cheaper.
This is all getting a bit silly so I'm going to borrow a EOBD lead off a friend at work and see if I can get any diagnostic codes out of the power steering controller. This assumes I can find the EOBD socket - while the regs state that it must be near the steering column, the regs don't say anything about it having to be easily accessible and on the MiTo I think it's hidden behind part of the trim.