Am I right in thinking APCOA don't pursue these?
They don't litigate, but if you ignore you'll have to deal with the minor hassle of incessant debt collector letters for several months.
If you appeal online, as the registered keeper only (no admissions as to the driver), with the below, they will almost certainly cancel the charge:
Dear Sirs,
I have received your Parking Charge Notice (Ref: ______) for vehicle registration mark _______, in which you allege that the driver has incurred a parking charge. I am appealing as the registered keeper of the vehicle. There is no obligation for me to name the driver and I will not be doing so. I note from your correspondence that you are not seeking to hold me liable as the registered keeper, under Schedule 4 of the Protection of Freedoms Act 2012 ("The Act").
Even if you were seeking to do so, as London Stansted Airport is not "relevant land" as defined by The Act you are unable to recover the charge from me, the keeper.
I am therefore unable to help you further with this matter, and look forward to your confirmation that the charge has been cancelled. If you choose to decline this appeal, you must issue a POPLA code.
Yours,