The Court held in C-102/15 Siemens just before mine and their summer break. It had escaped my attention. At issue was whether debt arising from the unjustified repayment of a fine for infringement of competition law falls within the scope of application of the Brussels I Recast. It does not. The Court distinguished flyLAL: while private actions brought to ensure compliance with competition law fall within the scope of the Regulation, a penalty imposed by an administrative authority in the exercise of the regulatory powers conferred upon it under national legislation comes within the concept of ‘administrative matters’, excluded from the scope of Regulation No 44/2001 in accordance with Article 1(1) thereof.(at 35).
An action in unjust enrichment related to the interest due, following to and fro, imposition and rescinding, ending finally in confirmation of the fine, is intimately bound up with that fine and therefore follows it in the exclusion.
A judgment of note for those who wish to keep complete overview.
(Handbook of) European private international law, 2nd ed. 2016, Chapter 2, Heading 18.104.22.168 ff.