£790 is very cheap

Depends on the type of conversion.

A "single mixer" conversion, just stuffing a gas mixer into the inlet tract, could easily come in at that sort of price. You probably went for the full monty and had LPG injectors fitted, along with a "piggybacked" ECU to run the thing on gas and feed the "real" ECU with dummy information to make it think everything's working normally. As this requires (at a minimum) the removal of the inlet manifold to drill for and fit the gas injectors, it can cost quite a bit.

The single mixer type won't be as fuel efficient, is unsuitable for more modern vehicles with full electronic engine management and can have a nasty habit of going <BANG>. The latter is particularly common on conversions to older Land Rover V8s (Discos, Range Rovers, etc), where many cheaper conversions placed the mixer in the conveniently placed intake duct, upstream of the air filter assembly. This provides a large volume of air / gas mix to detonate in the event of any backflow from the inlet manifold and usually results in the destruction of the air filter box.

Could we please stop mentioning Polly Toynbee? It's not doing my blood pressure any favours.....

Valvecare or to be a bit more proprietry -flashlube, or any one of half a dozen 'valve saver fluids.

OK from the top a 2002 Citroen Xsara Picasso - 1.8 an egas kit - no problems ran for years -no valvesaver fluid. Part ex'd against the Note.

2003 Xtrail 2.5 SVE Auto - egas kit converted four years ago just fitted a driptube valvesaver kit after 25K lpg miles. No issues. - still owned.

These were fitted by a local chap to me for £1600 each. But he's lowered his prices since then.

2010 Nissan Note 1.6 auto - after 23K miles just fitted a driptube valvesaver kit (thats the £30 one not the £200 electronic version). Kit is Ac stag 400 (I think). Kit was fitted at 75 miles now done over 25K - no issues no problems. Yes it was cheap but the todays price is only £1050 (incl vat) so still cheap. He occasionally does an ebay auction for conversion starting at £1...and I won one! I recommended him to HJ and his details can be found here...

http://www.honestjohn.co.uk/faq/lpg-and-cng/

PJ Cars are mentioned at the bottom of the page. LPG makes a lot of sense for me as I do around 25 K miles a year. Now whilst I have just fitted flashlube to 'the fleet' it's only through a bit of paranoia as I haven't seen any issues so far. But at £30 for the kit and a litre (10K miles worth) of fluid it seemed to be cheap enough to just do it anyway. Flashlube are the only ones making a electronic kit which does cost £200 fitted. My understanding is that as a rule (there are exceptions) all german cars just don't need it (esp MB). Some japanese cars reportedly do. I think there's a great deal of FUD about the issue and it's really just another lottery if you get VSR or not. So far so good (crosses fingers).