Taking into consideration everyone's comments, and my actual situation I have decided to go with the two small gas tanks on the roof and my fire extinguisher is going to be bolted to the floor under the passenger seat (I've realised that the release catch is fairly easy to operate).

I am a 'just in case' person; after all you don't insure your home, car or property expecting to need it, you insure it 'in case you need it. 

(I also hope I never actually have to use my snake bite bandage either lol).

Anyway, so now the roof rack has arrived I have to figure out how to attach the tanks to the roof, eg base plate and straps. Super Cheap Auto advertised a tri strap thingy for smaller tanks, that looked like it might work but surprise surprise because of black Friday sales they are sold out so I can't even go and have a look to see if they would work.

Does anyone have pictures of what a small base plate, straps (process) as I'm a very visual person