Hello Dave,
I agree with Jon that you should look first at the shock absorbers/dampers. Try comparing the ease of compression and rebound between your tourer and your V6. That might reveal a clue.
Incidentally I couldn't disagree more with what you said about the Rover 75 suspension. Mine is supremely comfortable, but then I don't have fashionable ultra low profile tyres which give a harder ride. What's more, every woman who travels in the passenger seat remarks what a nice car it is and one of those owns a BMW!
Simon