The airline is responsible for the damage. You have two avenues to go down. Start by writing to
Passenger complaints
Civil Aviation Authority
CAA House
45-59 Kingsway
London
WC2B 6TE
Or call 020 7240 6061.
The key is to focus on the reasonableness of your actions. The fault was not apparent until you had already left the airport building and you reported it at the first opportunity after that. I suspect Aegean will cave at the CAA level, but if they do not you can sue in small claims. It is very easy to file, you can do it online, submit a statement and your evidence. You will win...and Aegean will have to pay or face you arriving at Heathrow with a bailiff and TV crew ready to impound a 737.
https://www.moneyclaim.gov.uk/web/mcol/welcome