Phew - if ever there was a good time for your pedal spindle to separate from your pedal this was it, with a nice wide bus stop to attempt a left footed dismount, while my right foot hovered in mid air with the pedal still on it!
Having snapped the pedal from the shoe, it looks like it simply unscrewed. So I screwed it back on to the spindle and did the last mile home.
As it happened quickly I'm not sure if I've lost bearings. The pedals are Campag Chorus look pedals in silver, would anyone know if they have ball bearings ? Am I okay to simply tighten up and carry on?