Last time I was buying diesel at Wheaton Aston, I had a nosey at the splits being declared by previous customers (they keep it all on a list) and they varied between 100% red to 40% red / 60% white. This is going to make a big difference to the price. 100% red will cost around 78p per litre somewhere cheap. You could be paying £1.30 a litre on a 40/60 split.
To be on the right side of the law, you need to be able to justify what you've bought. If you have a permanent mooring and NEVER move your boat, you should be ok on 100% split. If you continuously cruise, you'll need a very convincing reason for doing the same if HMRC comes knocking. Perhaps you have a pedalo system installed in your boat and propel yourself that way?
The question is, what is the likelihood of HMRC checking up on you? On past evidence - practically zero. I'm sure that makes it tempting for some to declare 100% red to save a few bob. For myself, I always declare a split, even if it's small. Then I feel a bit better about not risking being prosecuted for tax evasion.
Good point. I'd always assumed that the 60/40 splitters simply didn't understand how the system worked and were scared that they could be prosecuted for selling anything else. Unlikely as that sounds.
To be honest, I'd rather HMRC didn't look at anything to do with the canals, marinas or otherwise. I can only see it leading to unforeseen and unwanted issues. As it is, I simply don't buy from anyone who enforces a split on me.