Hi, yes I service Prins systems.
I currently have the interface and software for Prins VSI1 systems but not for Prins VSI2 systems. If yours was fitted more than a year or so ago it will almost certainly be a VSI1 system so no problem there.
Seems your fault is most likely just a calibration issue, in which case easily sorted, but I have some Prins spares here anyway. Service includes the check/adjustment of calibration and would be £60 for the Prins system.
I think you'll find my price a lot cheaper than most Prins dealers price and that I'd be more likely to fix the problem. Most installers that are not Prins dealers tend not to have the facilities or know how to diagnose / repair / calibrate Prins systems... I wouldn't fancy your chances at an installer you might be pointed to by moderators on the Lexus forum. http://www.lpgforum.co.uk/viewtopic.php?f=28&t=14184
Website could do with a bit of refreshing to make it look more up to date and fix broken links etc but all info on it is as relevant today as it was in 2012. Bit of a chore to update it as there's so much info on it etc and I do all the work myself. I wish more sites were done by the site owner rather than by them employing web site professionals... On too many sites it's obvious that most of the text was written by a website pro, and where the LPG pro isn't great at explaining things some website pros seem to copy content from other websites. A refresh for my site might involve a gallery etc, since I've only recently started taking pics (when I remember!).
Full time LPG installer
Servicing / Diagnostics / Repairs to all systems / DIY conversion kits supplied with thorough tech support
2 miles A1, 8 miles M62, http://www.Lpgc.co.uk