From what little i can find on the topic, it would appear all you need is a tested and current tank.
I would think that as the vehicle is not subject to any form of registration, WOF or certification as its not for use on public roads, logic would suggest it therefore doesn't require LPG certs or WOF's.
The catch may then be, that if you rock up the servo and go to fill you trials vehicle thats sitting on the trailer, an attendant may go looking for the LPG cert or WOF, and on not finding it may refuse to allow you to fill the vehicle up.
The way i see it is that, on a non road legal vehicle, provided you have a LPG tank that is tested and still current, then its no different to heading in to the servo with your BBQ gas bottle or gas heater bottle and getting it filled.
Is that how it works????
Where's Mark BlackCruiser, he run's LPG.................
