Andy,
Don't know if this helps but when UK sources talk about gallons, ' gallons' is short hand for the grandly named 'imperial gallons'. There is also the US gallon which is a lesser unit, so different info might depend on the source of the website.
As a guide, 11 imperial gallons is equal to 13.21 US gallons; 11 imperial gallons is equal to 50.01 litres 11 US gallons is equal to 41.64 litres
Regards
