Now I have had advice from the RYA legal department.

The RYA Guidance Notes are at:

http://www.rya.org.uk/NR/rdonlyres/F36E0464-90A2-44B5-84B4-2C8CCB1027F6/0/CARRIAGEANDSTORAGEOFPETROL.pdf

I have now spoken to someone in the RYA legal department and they confirm the following:

• Different regulations apply to transport of petrol in a car if the fuel is going to kept in the car or if it is being transported for use elsewhere... such as on a boat.
  
  The following applies to me in that petrol will be transported in a car from a garage to the boat and then immediately transferred onto the boat and NOT stored in the car or elsewhere.
• Largest metal container - 23ltr, must be fit for purpose and appropriately marked
• Can use a purpose-made, proprietory, plastic outboatd fuel tank - Max 30ltr
• There may be local restrictions so I must check with each garage prior to departure
• There should be a suitable (powder or foam) 2kg extinguisher in the car and the driver (or other occupant - i.e. ME!) properly trained and able to handle any incident.
• On the boat I can have a full main tank, a full aux tank providing it is connected to the engine AND a second aux tank, up to 30ltr not connected to the engine.

On that basis I can take just 2 x 30ltr outboard tanks, one permanently connected. This will give me:

• Main: 136 ltr
• Aux 1 30 ltr
• Aux 2 30 ltr

Or a total of 196 ltr

Ideally I would devize a sensible and safe way to permanently attach two of these together to the engine so that I could then carry a third and that would give me 226 ltr on board and a better way to refuel as I could then carry all three to the garage and get 90 ltr in one car trip.

If anyone has an idea how to connect two outboard fuel tanks to a single engine please let me know.