All today the Vodafone SureSignal device (which was working yesterday to my premature amazement) has been down.
Five minutes ago it acquired synchronisation and I now have a 3G signal again.
I wonder why I'm not surprised.
Update Thursday: the 3G signal has now been up for 24 hours and seems stable. I'm guessing it took this long for Vodafone's back-office systems to sync up.