Pete - good advice from Fenlander there!
Chances are you already have another vented petrol tank in that shed - think lawnmower. The vent should be opened only the VERY smallest amount (do this when it's under slight pressure and as you open it you can stop at the very first release). This will allow equalisation but very little petrol vapour will follow after that. Logic says to store the tank in the coldest lowest part of the shed out of direct sunlight and away from sources of ignition (heaters, furnaces, switchgear). I keep mine beside my DIY cider "cave
" - what could possibly go wrong?