Thank you for all your responses.
I usually try and book direct with BA, but the flight was unavailable on BA.com and ~£100 more expensive on AerLingus than Expedia.
I've already had several frustrating phone calls with Expedia and see no point in having further. My second phone call was in fact to the BA gold line, where a very empathetic and friendly lady directed me to Aer Lingus.
I have now sent my final reply to Expedia quoting EC261 and the option of MCOL if no success within 10 working days. I have to say that I've spend too much time on this already and have no appetite for legal proceedings - a call to Amex is very likely the next step.

The experience dealing with Expedia has not been a pleasant one. I've booked a couple of TP holidays in the recent QR sale with priceline for next year. Now I've gone a bit off online TAs and hope I won't regret this.