Well I don't suppose there's much chance now of getting any money out of Rover to fix it for you...
I had similar problems in a '98 Mercedes E class (though it had done a few miles more
). I'd suggest finding a specialist auto gearbox workshop. They should be able to diagnose more accurately - could be it just needs an ecu, and if it does need a gearbox they may be able to overhaul it or supply a reconditioned one much cheaper than full dealer price!