Ian the reply is from one of our berth holders who decided to ask the Douane the question we are all wondering.
As I have said before we went to Belgium two years ago and were ignored by customs but the thought of a fine at over 4 euro a litre based on the tank size (500 litres in our case) is enough to dampen my enthusiasm; much as I love Belgium.
The Dutch examples (from memory) were Yotties carrying spare cans of Red more i.e. carrying spare fuel in addition to their tanks. Having these strapped on to the upper works would be a Red rag one would think.
The letter from the Douane states "We have required information from the Dutch Customs about their policy concerning red diesel and received the following information from them" so the Douane have consulted and got the response "You can fill up your tanks with red diesel, but not take more fuel with you then the amount of your tanks. You have to be able to show the receipt, which can’t be older than 1 year old. On the receipt should be written that “the difference in tax between white and red diesel is paid by the UK Government (the 60/40 notice)â€. According to the Dutch Customs this should be on the receipt automatically."
I would suggest if you are considering a trip you write and gain your own response to satisfy yourself and know what you need to adhere to.