golf gti problem

i have a golf gti 8valve m reg.my battery went flat and i purchased another battery.i had it fitted and all was ok for a time.then it broke down again.im getting a CErr message on the computer and the milage on my speedo has reset to zero.the rac man said it was a message problem between the key and the factory fit immobiliser.it just wont go off and the car runs and starts and drives great but for this damn CErr messsage.can anyone help?????
email me on [email protected] thanks andy.:confused:
the car also has a total closure scorpion alarm system fittted