Xsara Picasso faulty remote locking

I have a 2002 xsara picasso 2.0 hdi, the remote 'blipper' doesnt activate the central locking.The locking works with the key manually.
I have replaced the battery but it hasnt worked, the remote doesnt have an LED so it is hard to see if the problem is with the fob or the car. I would like to fix this myself rather than pay a dealer!