"What's the price of diesel on the 60/40 split" I asked...

It does seem to me that 'what's the price of diesel / gasoil on the 60/40 split'... is a very ambiguous question. Might not 'what will you charge me for 100 litres of diesel if 40% is for domestic services?' be a better way to phrase the question?
 
And the other strange thing is.... That 61.7p per litre at 0% tax you were quoted.. It will be 61.7p whether they are selling to you at 60:40, 40:60, 75:25 or another other ratio! It is not relevant to the declared purchase ratio.

And I would say you were right in the way you asked for the price per litre at a certain ratio(since most marinas I have been to will split in any way you declare for and will also quote for that ratio).

PS - At your place in Cowes - it appears that you would never have got the fuel for 61.7p/litre unless you had declared a 100% heating / 0% propulsion split. I guess you might have been able to do that if your main boat power was a petrol outboard and you had a diesel Eberspacher!
 
Top