Good morning Mike, I tried stopping apache and then starting it again, following your suggestion. I still see the same error with http://thesolentmetropolitan.com afterwards.
rob@server03:/var/www$ sudo service apache2 stop
rob@server03:/var/www$ ps -ef | grep apache2
rob 530313 530140 0 09:09 pts/0 00:00:00 grep --color=auto apache2
rob@server03:/var/www$ sudo service apache2 start
rob@server03:/var/www$ ps -ef | grep apache
root 530328 1 0 09:09 ? 00:00:00 /usr/sbin/apache2 -k start
www-data 530330 530328 0 09:09 ? 00:00:00 /usr/sbin/apache2 -k start
www-data 530331 530328 0 09:09 ? 00:00:00 /usr/sbin/apache2 -k start
www-data 530332 530328 0 09:09 ? 00:00:00 /usr/sbin/apache2 -k start
www-data 530333 530328 0 09:09 ? 00:00:00 /usr/sbin/apache2 -k start
www-data 530334 530328 0 09:09 ? 00:00:00 /usr/sbin/apache2 -k start
www-data 530336 530328 0 09:09 ? 00:00:00 /usr/sbin/apache2 -k start
www-data 530337 530328 0 09:09 ? 00:00:00 /usr/sbin/apache2 -k start
www-data 530338 530328 0 09:09 ? 00:00:00 /usr/sbin/apache2 -k start
rob 530350 530140 0 09:10 pts/0 00:00:00 grep --color=auto apache
rob@server03:/var/www$
Also, I have started asking about the issue on serverfault because I do appreciate that it is outside of the LetsEncrypt remit as you rightly advise. I want to be respectful in disclosing/advising of all the help I'm asking for.
Moreover because I'm really stuck, it is a bizarre issue.
One of the other sites is nearing its LetsEncrypt renewal date on 26 March, internationalgospelchoir.uk so to stop that running into similar problems with renewal, at the moment the only solution I can think of is that I'm going to have to build a new Digital Ocean droplet this weekend and move that site to it. I could still keep the current server for ongoing troubleshooting - it's worthwhile me trying to pursue that because the problem could come up again and finding the root cause could benefit others as well I would hope.
Thank you for your further follow up.