There were several landmark court cases in the UK recently where delays arising from technical faults were found to not be extraordinary circumstances
https://en.wikipedia.org/wiki/Huzar_v_Jet2.com
It's not entirely black and white however, some issues can be considered extraordinary, the failure of essential flight controls before departure being one of them, or damage caused by a foreign object. Of course in this case we don't know what the issue was.
There are many flight delay solicitors out there who operate on a no-win-no-fee basis who would be happy to help I'm sure. EI may try and bluff if you deal with them directly but their evidence has to be able to stand up in court.