how to sort problem with immobiliser on my p reg MGF?

my p reg rover mgf has just been in the garage .while there they had a problem with the car not starting due to the immobiliser they remedied the problem by uncoupling the batterie for 24 hours which worked to enable me to drive the car home it has only been stood for a couple of hours and the problem has reoccured how do i fix this i have no handbooks or key numbers has these were stolen acouple of years ago