Under EU law, with a delay of that magnitude, carriers are obligated to make compensation payments to passengers of statutorily fixed amounts. I believe the amount in our situation was €400. Of course, Aegean (our LHR-ATH carrier) tried to weasel out of this by offering alternative compensation (a one-way ticket, which would definitely be valued at lower than €400). They are obligated to offer the statutory amount if you ask, but they aren't required to advertise it. Luckily, when we got home after our trip, we contacted Aegean and did get the full amount without major headaches.