I have a spare ecu for my Discovery TD5. is it possible to set it up as "plug and play" somehow? maybe it is possible to disable the immobiliser function of the BCU? i tried turning the passive immobiliser off and connected the spare ecu. copied the injector codes from the original ecu but it wouldnt fire

All you have to do is to make it learn the security code in ECU utility menu then once learned it will be plug and play, don't mess with the BCU's settings