Hi @JonCatmull
tls-sni-01 is dead, support ends 2019-03. So use
sudo certbot --nginx --dry-run --preferred-challenges http -d mytownrocks.co.uk -d www.mytownrocks.co.uk
to switch to http validation and to create a test certificate. If that works, remove the --dry-run to create a correct certificate. If that doesn't work, share the output.
PS: You should always create one certificate with both domain names (www + non-www), if both names have dns entries. So both connections are secure.