Allow me to simplify things a whole lot:
sudo certbot certonly --cert-name mail -a webroot -w /var/www/html -d mail.2e0epv.com,mail.kylebrown.co.uk
This will create a new certificate named mail. Once you successfully receive this certificate, you can use certbot certificates to list all of your stored certificates and note their names then use certbot delete name to delete those unneeded certificates.
Looks like a python error now, @rg305. Since certbot is generating the CSR, so the PEM should be fine in it.
Yeah bad turned to worse really fast.
But it might have been the underlying problem all along... hmm...
I suggest he update the client.
Wait... the cert... succeeded? 
I see it now. You have corrupted certs in your store.
Now there is "mail" and "mailcert" ?!?!?!
What happened to: --cert-name mail.2e0epv.com
Do we really care?
Please show the entries in the /live/ and /archive/mail/ and also /archive/mailcert/ folders.
ls -la /etc/letsencrypt/live/
ls -la /etc/letsencrypt/archive/mail/
ls -la /etc/letsencrypt/archive/mailcert/
and also show:
df -h
After what @rg305 asked, please run the following:
sudo certbot delete --cert-name mailcert
sudo certbot delete --cert-name mail.2e0epv.com
Good catch @rg305.
If that fails try it like:
sudo certbot delete --cert-name mailcert
sudo certbot delete --cert-name mail.2e0epv.com
What about:
Now...
sudo certbot certificates
Just for info:
That probably needs some expanding and correcting, since you have deleted certs:
ls -la /etc/letsencrypt/live/mail/
ls -la /etc/letsencrypt/archive/mail/
df -h
Beautiful! 
Ok, now now I have a cert now do I need to setup a server block in postfix, dovecot and nginx?
I would think they all operate differently.
You need to find the "how to" use a cert with each of those (individually).
This issue has now been solved! 
Ok, then I will stop asking for the "ls -la"s and the "df -h" - LOL
But I will ask: How did you get it fixed?
[so that anyone who reads this topic can benefit]