A couple of the main WFP suppliers sell expensive diesel fired add-on water heaters as they are the easy option as van insurance issues generally do not arise.
However, whilst many DIYer's have opted for the cheaper option of gas fired heaters, there is still the issue with regard to gas bottle storage in the van, procedure and a proper exhaust facility. Many shortcut the exhaust issue by mounting the boiler on their back doors and leaving them open to vent the boiler.
Peter Fogwill has a boiler on Ebay (or did have) for about £190.00 excluding delivery, and gas bottle, and was expecting to sell a complete DIY kit with insurance approval by the end of January 2011. It's now the end of February - nothing. I'm sure there are more issues to fitting an approved gas heater system than it first appears.
The gas carrying rules for caravans are easy to comply with. A gas bottle on the front drawbar mounted outside the living/sleeping quarters vented to the atmosphere is what is required. The same rule applies to motor homes. They have gas bottle storage in a compartment that is sealed to the inside, vented and only accessible from the outside.
This is where the problem is - carrying a gas bottle, even properly secured, inside the van goes against the regulations.
It is less of a problem if you have proper LPG tanks fitted underneath your van, but there is a ground clearance requirement - I can't remember the figures, but I couldn't fit tanks under the chassis of my Citroen Relay as it sits down too low when my 650l tank is full. Most tanks that are fitted underneath are fitted to 4 x 4's which have high ground clearance.
You could have a tank fitted inside your van by an approved installer, but these are usually designed for fuel guzzling petrol engines so tend to be quite big and take up a lot of valuable space. Theses tanks are designed to have sealed venting covers over their fittings that will direct any escaping gas to the outside of the vehicle. Both under chassis and internal tanks have a life of 10 years and they then need to be replaced then. They will also need an annual inspection that they still meet safety standards and a certificate of compliance is issued by an authorised person.
The plus to having an LPG tank fitted is that they are filled at your local Autogas petrol station and the gas is cheaper than the 'bottled' gas.
Oh, just because you have a properly fitted system by an approved staller doesn't mean that you current insurer is guaranteed to insure your van with it.
Spruce