Thanks for your response.
To give you some background I have inherited this site so still finding my way around it.
Looking at the 'Page Loading Info', I see that some CSS and JS are blocked in my robots.txt file. - this could be the reason for the incorrect rendering because the directives to 'format' the page for small screens (ie the responsiveness) are in those CSS files.
https://search.google.com/test/mobile-friendly?id=QBp2hSFGf82SQQhysqHwHQ&hl=en&view=fetch-info
My current robots.txt file is:
Disallow: /media/plg_jchoptimize/cache/
Allow: /.js
Allow: /.css
Allow: /.png
Allow: /.jpg
Allow: /.gif
Allow: /components/.js
Allow: /components/.css
Allow: /modules/.js
Allow: /modules/.css
Allow: /plugins/.js
Allow: /plugins/.css
Allow: /templates/.js
Allow: /templates/.css
Allow: /component/osmap/?view=xml&id=1
Allow: /component/osmap/?view=xml&id=1&images=1
Allow: /components/com_sppagebuilder/assets/
Allow: /media/
Allow: /plugins/system/jcemediabox/
Allow: /libraries/.js
Allow: /libraries/.css
Disallow: /?
Disallow: /=
Disallow: /administrator/
Disallow: /bin/
Disallow: /cache/
Disallow: /cli/
Disallow: /components/
Disallow: /includes/
Disallow: /installation/
Disallow: /language/
Disallow: /layouts/
Disallow: /libraries/
Disallow: /logs/
Disallow: /modules/
Disallow: /plugins/
Disallow: /tmp/
Sitemap: https://www.document-genetics.co.uk/sitemap
Should my robots.txt file simply be the standard recommend below:
User-agent: *
Disallow: /administrator/
Disallow: /bin/
Disallow: /cache/
Disallow: /cli/
Disallow: /components/
Disallow: /includes/
Disallow: /installation/
Disallow: /language/
Disallow: /layouts/
Disallow: /libraries/
Disallow: /logs/
Disallow: /modules/
Disallow: /plugins/
Disallow: /tmp/
Many thanks
Steve