Don't get in a state about it, really not worth it! I notice alot of cars (even a new Audi up my road) with bits of condesation in the headlights. It has been very damp for a long time now in (I'm assuming you are in the UK) and unless the car is used everyday it's easy for condensation to build up. If it were just one headlight, I'd be more concerned the unit was faulty rather than it being doe to prevailing conditions.

As long as it is condensation of course, and not 'standing water' which may point to something more serious. Don't panic, but are you confident there is no other sign of water in the car? Under the carpets for example, any smell of damp (or conversely too much air freshener) etc, anything to suggest the car may have been the victim of flood immersion?

There must be loads of pre-reg Focuses (Foci?) around that are for sale.

Just look for one that doesn't have condensation in the headlights!

All modern headlights are ventilated through small holes at the top and bottom. Ideally, they allow a gentle flow of air through the enclosure. If there was no ventilation there would be no way for the condensation ever to disappear. However, this flow of air can also allow moisture to enter the headlight. And then the same thing happens to the glass in the headlight as happens to the windows in a car when it’s cold outside and warm inside. You get a thin film of condensation. So basically when you start driving the car it will be gone!