I might have solved it. @Byron Dickens Yes, I had downloaded the drivers and the Focusrite control and I had updated the firmware for the interface, all before opening CbB.
The issue seems to have been that I did not select 'show mono inputs' in preferences > Audio > devices. Once I did that (having re-read the manual - I genuinely had printed out all 20 pages prior to the unit arriving), Focusrite USB ASIO showed up.
Stephen Power's post in SOLVED - Focusrite Scarlett Solo 3rd Gen & Cakewalk - Driver? was marked as the answer