2004 citroen dispatch central locking problems?

Hi all. Recently purchased a 54 Reg dispatch. Use the key in the drivers door and it locks or unlocks all doors. Button on the key does nothing at all.
So I've purchased an aftermarket central locking kit, same as what a friend of mine purchased for his van... (same van as mine) and his works fine. Wired mine as per the instructions and same as my mates but it does not work. Can hear the new kit clicking as if it's trying to lock or unlock the doors but that's all.
Any ideas please or is there another device for the central locking which could be at fault?

Thanks.