I have a DONE3, the last two sectors of which are HKG-LHR-JED on CX and BA.

Originally, at the time of ticketing, LHR was just a transit of a few hours. I have since turned it into a stopover. CX is charging me GBP183 as "we need to collect UK departure tax". By my calculation I should only be charged GBP134 which is I understand to be the APD appropriate to my journey.

Does this sound right? Is CX trying to get more out of me?