how can i recode my citroen xantia key fob?

i have a citroen xantia 2.0i r reg and the key fob worked ok but car has been stood for some time and no longer works on the remote locking so i cant arm the alarm. when the button on the fob is pressed the red led lights up on the fob and iv replaced the batteries. the led on the dash for the alarm is constantly lit very dim u can only see it at night.the infered dome has been replaced too