Apparently it is not a common problem on the X-Type, but a few cases have been reported on the S-Type. Here is what the mechanic told me -
Warning lights, rev counter dropping off and a blank mileage display told me that the vehicle had lost its CAN communication network. The control modules use this network to pass information which also includes the security ID which will stop the vehicle from starting.
The immobiliser system on this vehicle is software based and has a number of modules all linked together and passing data between then, it is not based on one unit.
Here is an image of an X-Type dash with the problem