Dear Ditto, Often1, teahan & davidj1,
Thank you kindly for your responses.
I hadn't thought of a claim agency, so thank you for this idea.
I spoke to Ryanair chat yesterday and they said they would send me a reply within 24 hours. We will see.
They say that an operator cancelled the ticket sometime between my check-in and boarding, saying that I must have requested the cancellation. I asked for the source and proof of my requesting cancellation in that short window of time and of course there is no proof because it didn't happen! Urghh. I personally think someone behind the scenes has made a technical error perhaps. But I shouldn't pay for that.
I've now opened up investigations with AESA and with Aviation ADR.
Unfortunately, I don't really wish to take this to the media, for privacy reasons.