When I first got my TM supplied with Plastic marine petrol tank, I spent a long time getting to the bottom of the legalities etc. As Glynn correctly says about the limit of fuel allowed in plastic containers being carried in a vehicle. This is true, and any fuel carried ina vehicle in plastic containers MUST strictly be for the sole purpose of refueling the vehicles internal combustion engine. However in the case of a plastic tank for the purpose of fuelling a TM fixed in the van, this plastic tank DOES NOT come under the same regulations. In fact at present there are no definable regulations that cover it, it is a grey area that is not at present defined in legislation. The powers that be, once I finally got to speak to the official with the answers, told me that if the plastic tank was supplied with the machine ,it is okay, as this would have been all checked out by the department responsible for that area. My insurance company confirmed this and told me that all this area had been thrashed out before they initially decided to offer cover.
Also just to add. Last year I had to follow the police to a weigh station , when they thought I was overloaded. While I was ther they gave the van a thorough going over internally as well as externally. They never said anything about my 25ltr plastic tank, strapped to a fixed shelf in the back.
Dave.