I keep my yacht at Whitehaven marina and buy red diesel from the marina.
I have to fill in a usage form stating what I'm using the red diesel for and I have two options .
1/ for propulsion I have to pay the higher tax tariff (more than white diesel at a garage forecourt.
2/for heating I pay the lower tariff as normal red diesel prices.
It is illegal to decant fuel from external containers within the marina area.
So most of us when buying diesel at the marina office always say my heater is massive diesel heater and my engine is a wee small thing that burns very little fuel, and all we get from the office is a pleasant smile as they are very understanding good staff.
Of course when sailing over to Isle of Man or Ireland and Scots ports we never get asked what we using diesel for as it would be a silly question wouldn't it.