Welcome to the forum. My recommendation is to clean your Mass Air Flow sensor. It may not sound related, but on a Rover it probably is. Also check to make sure you have a good clean air filter.
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When working with GEMS vehicles. Never try to locate a misfire by pulling plug leads off - pull the injector connector instead. Otherwise you can blow up the ECU.
Secondly never swap ECUs between vehicles to test them, because both vehicles and ECUs may stop working, and you'll have to have them all reset by a franchised LR dealer (ouch!).