A number of British yachts have been fined for having red diesel in their tanks while visiting Belgium. This has happened before and the Belgian government stated it would not happen again but it has.
Update 3rd Aug 2017
As stated on the RYA web site ( link ) the Belgian government has said it will not fine British boats ie enforce what they say is European law until after an EU meeting on 18th September.
The CA ( link ) has also had an email from the Belgian Ambassador making the same statement. However, the CA also advises that 2 years (!) of diesel receipts should be carried with 100% duty paid, and Schengen forms ready.