Ford Focus 1.6 LX 2000 Reg Central locking?

The remote central locking on my wife's car has stopped working. I can lock the car manually from the drivers side, but if I try to use the remote it locks and immediately unlocks again. Also if I try to lock the car from the inside on the drivers side or manually lock it from the passenger side the same thing happens (the manual locking mechanism on the drivers side does not operate when the passenger side interior lock is used).

I should be grateful for any advice (apart from sell the car!)

Regards