The air filter was squeezed to fit in its cover and was found to be detached from the inlet flange. The engine management light had come on and there was a big loss of power. A local garage replaced the filter and a sensor and the car has performed fine since. The dealer who carried out the service claims that the badly fitted filter would not have caused the loss of power. Seemed too much of a coincidence to me so I am looking for a second opinion.