Central locking not working on a 53 reg Citroen Relay 2.2 HDi?

Hi All
Recently picked up a 2003 2.2 HDi Citroën Relay. The central locking doesn't work using the remote key, either by remote or by turning the key in any lock?
I dont have a manual or wiring diagram and apart from checking the fuses, which are all fine, I'm not sure where to start?
Is c/locking controlled by a relay? Any ideas on where to start looking/testing?
Many thanks