Here's the link repeated from post 17 above:
The quantities that are permitted to be filled at a petrol station distinguish between 'Plastc Containers', 'Metal Containers', built-in petrol tanks and portable petrol tanks. The legally permissible quantities are additionally subject to any policy requirements of the particular filling station.
Having once filled what is permitted, there are different rules as to what can then be carried in a car or truck.
The above link should answer most questions and the advice in it could easily be printed off and kept for production if necessary.