Here's another idea - require all people who are paid to enter national public services (ie MPs, MEPs) to publish their tax returns for all countries in which they declare income for the rest of tier lives, with a 7 figure penalty for failing to do so, doubling each year. Although this would be an imposition for the majority of MPs, MEPs, it would show who does have their nose in the trough benefiting from the disarray caused by leaving.