Thanks very much all for your helpful replies.
When I spoke with an agent, they said they couldn't accept payment over the telephone?
I'm going to try once more on the app, which I haven't tried yet, but interestingly when I looked at it last night, it said there was availability, but only with double avios (flexi).
This may point towards the website displaying phantom availability as suggested above.
When I made a redemption booking last year, it all went through without issue.
All extremely frustrating indeed!