Well, as I hate threads which start a question and then go dormant without an answer, in the end I found this:
https://www.amazon.co.uk/UGREEN-USB-.../dp/B081GGB53R
For 12 quid!
Works perfectly.
Unfortunately when I was fiddling around with the EQ and balance settings on my phone I discovered the reason the sound seems quite gutless is that the mid+sub unit on the drivers side is completely dead, so I may have to replace that anyway....