Hi and welcome to just answer
As far as I'm aware the code should be the last one entered (your own code) before you changed the battery and can only be changed after entering that code, it does not reset to a factory setting as this would be to easy to overcome if the vehicle was stolen.
The only success I've had with these is to leave the ignition on for 1/2 an hour (don't try to enter the code), turn off then back on and enter your own code again as normal. If this doesn't work it will be a job for an auto electrician or a Citroen dealer.
It would be worth while phoning Citroen assist first even though the car is well out of warranty as they may be able to help although they will want to be sure they are talking to the vehicles rightful owner.
I hope this helps
\thank you for your response, i will try this now and let you know in half an hour.