I'm not really surprised...pax figures have rocketed since three flights daily were introduced and the new increased tax on long haul flights will surely encourage traffic via BRU/AMS/CDG and DUS, this leg just having a £1 increase.
The tax will be the same as a direct UK flight as it's determined by the end destination if booking an E-ticket, Unless you spend more than 24hours in the hub european city before onward travel.