There are four big springs, one at each corner, which allow the car to bounce on its wheels. It transpires that one of these, the one at the front on the driver's side, had broken. They've ordered two, so that they can replace both of the front ones for symmetry, and they should arrive in the morning. I should be able to collect the car in the evening.
And it's all under warranty, too. Apparently they expect this sort of thing to last more than eighteen months. So that's good.