Hi Chris,
familiar story here, I also converted a Omega V6 to LPG and ran it for 200,000 miles with few problems, then asked Gastech for advice on a fresh motor, GS300 they advise so off I went buying a 2002 Se (same engine as your Sportcross) and had it converted with Flashlube. Complete disaster, numerous breakdowns ( I can never forget complete shutdown in lane 4 of the M25 near Heathrow) and 2 engine rebuilds later I gave up and bought a GS450 hybrid for work and a Sportcross to take the dog out, at last reliable cars again. Because the Omega ran so well or so long I am sure Gastech fit good kit and know what they are talking about but the GS NEVER ran sweetly on LPG. Hope this helps you to avoid making the same mistake as I made.
Regards Tony ( worried for the health of your Sportcross )