Solution 1:

This does work:

  • Navigate to Google Domains (https://domains.google.com)
  • Select the domain you wish to manage
  • Click the DNS icon
  • Scroll down to Custom resource records
  • Fill out the fields above the column heads / by the Add button (below is example data; amend per your needs)
    • Name: _acme-challenge.www
    • Type: TXT
    • TTL: 1h
    • Data: ABCDEF blah blah blah
  • Click Add

I'm not sure why this didn't work the first time I tried it as I think I did exactly the above; so putting this down to a glitch. Recording here in case it's of use to others searching for similar.