Solved (hopefully!). I've finally bought one, a Flexson Analogue-to-Digital Converter for Sonos Playbar (and similar devices).
The ADCs that Amazon showed me (priced between about £10 & £15) didn't include enough detail to be sure about the types of digital output, and where the jpower came from (USB would be difficult with our setup). Results from the wider web seemed to be either re the opposite conversion, or for the production side of music (serious-looking gear, doing a lot more than we needed and priced accordingly).
Remembering the knowledgeable Richer Sounds people in a forum they run here in ROI, I first tried their website (without much hope), and there was just one ADC there, the above Flexson model. That seemed a good sign, and the £30 price suggested higher quality than I'd seen on Amazon. And unlike those brands, Flexson had a web presence (specialising in accessories for Sonos), and the detailed specs included optical and coaxial outputs, and mains power, so that was problem solved (it'll probably arrive tomorrow).
There's a Richer Sounds shop in NI, but it's a day's work to get there and back (and delivery's quite fiddly to ROI), so I tried an Amazon search on the model, and there it was. Maybe the ones it showed me before were sponsored or otherwise manipulated results (they were followed by increasingly irrelevant ones).
As searches go, that felt like quite a marathon. But a helpful (and delighted) review I found of it was by a customer who'd spent 8 weeks trying to find something that would connect his stereo to his new Sonos soundbar, so I can't complain!