One small stab at it.
Not that I know that much about Z cars, but I had/have the same problem. It's not 100% fixed, and I am still getting under 14MPG (up from 9!). Anyways, I replaced all of my fuel injector connectors, and that helped a LOT! No more smoke from the tailpipe, and passed emissions easily. Also, my throttle switch was damaged, and so it was replaced. Now as for the bad mileage....
Nathan