I would have a question considering this, too.

Just upgraded to F using avios for a flight to PEK this week (BLQ-LHR-PEK HKG-LHR-BLQ in CW) and was asked for 230something Euro in higher taxes. Well...i haven't thought much about it as it is still "cheap" for an F flight and it was the last seat available, but i does seem a bit excessiv after reading here. How can i check if thats right? I paid originally Eur 464 EUR in supplements and taxes, a dummy booking of today for the same flight in F (so in contrary to my real booking the return leg is in F too) it shows me 567 EUR in taxes on a much higher fare.

Thanks already

Edit: i must add that my original flight from blq to lhr was cancelt and i was rebooked on an earlier flight which leaves now a bit more than the (taxwise) allowed 24 hours for my connection in london. Might it have something to do with that?

Edit 2:

I just saw the following position added on mmb which was not there on the original booking:
Air Passenger Duty - UK EUR213.55

So i guess it has to do with the going beyond the 24 hour period at my connection in lhr due to the cancelt BLQ-LHR flight?