That's pretty much what has happened to me, although my central locking wasn't working before I changed the battery.
I've had most of the interior trim out of the car to solve the leaking problem and on my way I've been looking for the sensor. It definitely isn't where the light is above the rear view mirror and I can't find it anywhere else!
To answer your battery question, it takes one battery and there should have been a type on your old battery, I think it is something like CR220 although I can't say for sure at the minute as I don't have my key handy. I'll post the exact type here when I have my key.
Hi, If you could let me know the battery type that would be great.
Since trying the suggestions on the forum i.e. holding down central locking switch and pressing fob, etc, have noticed that the car now locks when you drive off (anti intruder) which it didn't do before but still cannot lock doors with key fob. Would still like to operate central locking remotely however so any more advice would be appreciated - its a pain using the key especially in this rain!!!