I can recommend Hilton Autogas, http://www.hiltonautogas.com/
They did the conversions for me on a Ford Mondeo, and a few years later a Mercedes S Class. They use the Prins system. Not the cheapest system available, but generally recognised as probably the best. They are a few minutes off the M6 near Wolverhampton, and I have always found them to be very professional. If you phone, ask for Billy who is the owner (01902 866266).