I bought my car in July, it cost me £4,000 and it's insured for that sum with MoreThan. I then spent £1,700 on the conversion and contacted MoreThan to inform them of the conversion and to see what happened with my premium (conversion was done at about mid-term of insurance).
I was very surprised to discover that they give a 10% discount for LPG-fuelled (or dual-fuel) vehicles and that it could be given immediately (applied pro-rata). I also specifically asked about the cost of the conversion and what would happen in the event of an accident/whatever and was told that all the LPG equipment would be covered just as if it was factory-fitted original to the car.
I'm rather an 'appy bunny, I can tell you