Light prices surge

That's because the cost on a new car can be up to four times that of the equivalent unit for the previous model.

Our table shows the price difference for the UK's top 10 best-selling cars. We compared the cost of the dearest and cheapest for each car and that of its predecessor. In most cases, the former is found on the flagship, and the latter on the entry-level model.

On average, costs have increased by as much as 43 per cent. That's partly because many current headlight units are bigger - those on the Peugeot 207 sweep nearly halfway up the bonnet.

Improved technology is another factor. The costliest units for the 207 feature a directional beam and daylight sensor; those for the 206 don't.

And certain models, such as the Ford Focus, now offer xenon lights. Of the £377 you would pay for an ST unit, £163 is for the bulb. On the upside, modern lamps improve safety.