Bypassing Citroen Xsara (1.6i R reg) immobiliser?

Can anyone provide instructions as to how you bypass the immobiliser. The car was out of use for a couple of months, jump started it but it wouldn't start - had battery replaced, still won't start but now think it is immobiliser as that warning light is constantly flashing. Not worth going to Citroen dealer as I am selling car and the car isn't worth that much anyway.
Thanks