Thanks for all the useful replies, and apols for the delay in responding.
I am travelling on business, so all the arrangements must be handled through the agent (a major travel company). Suits me generally, as I don't have time to be doing it myself.
I was just intrigued to see if anyone else has had this experience. As it stands, the agent has sent my original full flex ticket for 'partial refund' and has paid for a one way economy ticket at close to half the cost of the original premium flex return fare. I am apparently now waitlisted for a prem economy seat, but by all accounts I will have to pay an upgrade fee against my new economy ticket to get this. That'll be an interesting discussion......
Thanks again.