Tax on tickets is seemingly an inpenetrable world - but are you aware that if you cancel a ticket that states 'non-refundable' you can still get the tax, fees and charges back?
Bit of info here off the Air Transport Users Council if this might gain a few brownie points and pay for the airport parking for the next time...:
http://www.caa.co.uk/docs/306/Press%...%20amended.pdf
'.....airlines rarely voluntarily refund taxes, fees and charges to passengers who do not travel. You have to ask......'