Well to provide an update in case it helps anyone else searching- when I called to book my return flight the agent said it looked as if it would have been considerably more economical to fly from INV. On calling back the next day to finalise (domestic flight was +1 day so had to be booked the day after I snagged the return long haul flights), they quoted Inverness for me and the saving was significant. They then slapped a charge fee on which I explained I was not happy to pay, given the only reason I booked ABZ was due to incorrect info. They took the day to check what I was saying and ended up satisfied I was given the wrong info in error so they waived the change fee. Happy days. Took a while but cannot fault anyone I spoke to after the first quite brusque lady that wouldn't listen to me explaining the APD issue.