OK after the wife's Aygo was trailered home the other evening I've been running it round the village and have seen the fault for myself. The speedo sometimes jumps about, too. About once in every ten starts the starter motor spins but doesn't engage to turn the engine. I've read on a Citroen C1 forum that this can be caused by a faulty earth strap so I'm going to take each one off and give it a good clean, maybe brush it back to bare metal, and see if that helps. Oh, and it's definitely related