So, anyway, my car, as generally trouble-free as it still is, does seem to get through a lot of remote central locking keyfob batteries (they're the little CR2032 watch batteries). Replacing these is reasonably easy, requiring only a bit of prising with a small flat-head screwdriver to get the keyfob housing apart, and of course the obligatory swearing. However, that's not sufficient to complete the job, because now you have to re-synchronise the keyfob with the car's internal electrics. This is actually very easy if you know how to do it, but it is rumoured that if you get a garage or a dealership to do it for you, as I'm sure some people do, they will charge you for the privilege.
Anyway, the second piece of advice on this page describes the correct method for re-synchronising a keyfob with a 2001/2002 Ford Focus; I can't speak for earlier or later models. Assuming that you've already done the battery replacement and swearing bit, here's what you have to do:
- Put the key in the ignition
- Turn the key from the OFF position to the II position and back four times in reasonably quick succession (the internet suggests about three seconds); you should then hear a beep and see the immobiliser light flash
- Press any of the keyfob buttons
- Turn the ignition to the I position and back to reset things
- Job done!