Hi
At around a £1.00 LTR from the marinas ,i use 2 ,5 gallon cans and fill then from the petrol station.
Every time i go down to the boat i put 10 gallons in,it doesnt take much to effort to keep it topped up,
By doing it this way i save around £100.00 on a full tank
As David says, around £1 per litre. But if you're over Yarmouth IOW way, it's the cheapest water side petrol around, 87p per litre last November. Not been there this year yet, but usually at least 10p per litre cheaper than marinas. Run by harbour comissioners, obviously false economy to make special trip.
I tend to refuel from cans as well. But beware petrol stations in Poole area tend to enforce petroleum regulations, regarding amount you can carry (22 litres) and type of container, must be metal if over 5 litres.