.....or something like this?

http://www.ebay.co.uk/itm/3-4-Gas-Me...MAAOSwImRYi90V

I reckon the supplier is ripping you right off mate, blaming air temps, tanks not vertical blaa blaa blaa. Can you vary the payments in the same way? "sorry it's a bit short this time but I've had my wallet on its side and I just can't get the same out of it"

Anyway does all this mean I can't fill my bbq from it anymore? :D

You can bring your butane bottles round anytime. 😁

could it be me topping up my D3 with LPG each time I visit that causes the differences? :eek: :D

Finally got to the bottom of this one. 27litres was right; it's all down to tank shape. The float gauge is linear while the shape of the tank is not so from the bottom to the middle it take more liters per centimeter of rise the higher the float lifts. From the middle up, less. Most of my Fill started around the 40% mark......