My L320 is going back together after being in a few bits and left with the battery disconnected. Can I get the immobiliser to switch off? Can I 'eck!

The fob operates the central-locking perfectly (so must be charged). The key turns in the ignition. The key opens the passenger door (which I thought would "prove" to the car I had access rights). The flashing light on the instrument cluster just doesn't stop flashing.

Have I missed a trick or does the solution involve an expensive laptop and a call-out?

TIA
Hugh