Is the ECU the one off the R75 or your Freelander?
Just talking about T16 engined Rover's for a moment; on the later cars with wasted spark ignition, the ECU (integrated with the immobiliser...) was coded to the flywheel I think. Doubt this would have been simplified later on.
You could conceivably need your ECU recoded to match your engine; however this is just a half-educated guess and I think you should check up on that before shelling out money.
Maybe talk to someone on mg-rover.org or rovertech.net if no-one here knows.
Good Luck,
Malcolm