Just tried this myself, sure enough as you scroll down to the card payment section on the final page of the checkout flow (after all the optional upsells, seat

Just tried this myself, sure enough as you scroll down to the card payment section on the final page of the checkout flow (after all the optional upsells, seat selection etc), the price increases by £10 x per passenger thanks to a "System surcharge" that is added. You wouldn't notice the charge being added unless you explicitly looked out for it, as it only appears when you start scrolling the page down to the card details form. Disabling ad block results in the charge not being applied. What's even more baffling is I had to complete a reCAPTCHA half way through the checkout flow, so them saying it's to do with bot detection is a stretch. Edited as I forgot to mention the captcha code during checkout flow.