Danny,
No offence is intended here, but I'm not sure what your knowledge of LPG installs is. If you like, you can give me a call, and I can talk you through the basic checks.
If I had paid that much cash for an LPG install that didn't idle right, I would be going nuts!
How does it behave at higher revs, when you give the loud pedal some abuse?
How bad is the idle difference on gas? Is it clearly noticable? Or just extremely slight?
Do you know where they positioned the injectors? This can play a fairly large part in idling so I hear.
If my car's not been used in a while (few days), it will have a really bad LPG idle until its had a good run. But then it's fine..
but 9.99 times out of ten, mine idles perfectly, and it ALWAYS without fail idles perfectly when hot
I'm no "expert" but it doesn't sound like the installers are either if they've let it go like that!
If you want to have a chat, then no problemo...
Am I right in thinking it's an OMVL dream kit?
I would also like to know where they picked up the RPM signal.
From the coloured wire on the DIS is not the best place, as the signal is not clean or consistent. Much better to get it from the relevent ECU pin wire.
Don't stress too much though, it's fixable, and if it runs OK on gas otherwise, it's not indicative of a major fault (but could go that way if the varorisor isn't getting a good coolant feed.
Looks like on a car such as this one, DIY may still be the best option if you know the car...