Originally Posted by drauk
...Or are you being charged for flying a > 2000kg aircraft IFR?
Yes, you are being charged in accordance with the rule that says if you're over 2 tonnes MTOW and flying IFR you incur charges based upon your weight/mass and your route.
The calculation is reasonably complex, with allowances within each crossed FIR for departure-phase and arrival-phase where there may already be a charge (nav charges etc.), and the route used by the calculator is the CFMU-ready route, notwithstanding that may not be the route flown
There is a charges calculator on the Eurocontrol website and if you go
HERE and follow some links you can download a calculator and the latest fee schedule for use in that calculator.
For a "typical" >2 tonne GA (say a 310 for example) twin routing IFR from the SE up to Glasgow, the charge would be about £80.