A new key fob, programmed for the car costs approx £100. However, Rover did plan for just this eventuality. In your owners info package, along with the handbook there should be a little card with a four digit code (not to be confused with the stereo code). This code is for disabling the immobiliser when the fob doesn't work. There are instructions on how to do this in the owners manual itself - it's all to do with turning the key a set number of times in the drivers door lock. Difficult to explain fully, but all details are in the owners manual - I had the exact same problem with my 1995 Rover 800.
Kieran, April 2007