i have done 2 diy conversions now, that doesn't make me an expert on the subject but i have a good idea of the pitfalls.
As Kevin has said DIY kits can be bought for about £650, i have just paid £630 for a multi point kit, 80l tank and a few other bits that i specifically wanted.
It took me 3 days pretty much flat out to do the job, now there are a few people who have seen my car and have commented on how 'hidden' the kit look, and this was the look i wanted, i could have taken less time and but i am not sure the install would have looked as good.
I have no doubt this chap in Cardiff can do an install in a day, but i would want to see the quality of his work before committing.
people in the LPG arena would have you believe this is a black art to an installation, there isn't, if you read the LPGA code of practice you will get a fairly good idea what you can and cant do.
Insurers - many insurers do not seem to increase premiums for LPG conversions, however you HAVE to tell them the car has been converted as this classifies as a modification.
You will lose about 10% of the engines power running on LPG, in all the time i have been running i have never noticed it, MPG will be down by about 5MPG, but remember you are paying nearly 50% less than you would if it was petrol.
If you have a cylinder tank then you would lose about a 3rd of your boot with an 80 - 100l tank, there are tanks that will fit in the spare wheel well but then are about 55 - 60l max and so may not offer the best range in the world, plus they come at a premium price as well.
hope that helps