2003 Citroen Relay 2.2 HDI central locking not working?

Hi All Recently picked up a 2003 2.2 HDi Citroen 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? I did the BSI reset thing but it made no difference. Many thanks

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