Off the top of my head, 300mm, although I may be wrong 
250mm is my recollection but I might be wrong.
I think it's one of those vague COP11 areas where they say "unless adequately protected", implying that there might be a way round it. I guess if you fitted it above the saloon tow bar in place of the petrol tank, for example, or built a protective guard around the tank, you could argue that it can't contact the ground as the tow bar / guard would contact first.
I would certainly be going down the route of binning the petrol tank to fit an off the shelf LPG tank, then a smaller custom made petrol tank to fit round it, than the other way round. Plenty of people will make you a petrol tank by TIG'ing a bit of ali together. Really not sure how you'd get on getting a custom made LPG tank.
