Sounds about right, I used to get 21mpg out of mine with normal mixed driving. They are utterly shockingly shite on fuel and has to be the worst performing car to mpg I have ever known. I then bought a mk2 st, even with it tuned to a little over 300bhp it was still more economical than the st170 was.

You could always check the air flow meter and lambda sensors are reading correctly, they are known weak points and can lead to poor mpg if not right.