Thanks for getting back to me.
Here's where things are starting to get a bit tricky...
When I made the first certificate I typed my email address incorrectly. I corrected it in the renew-certificate.sh and crontab files. But now, when I try to renew the certificates I'm getting error messages one which tells me that the email address doesn't exist and another which tells me that a certificate already exists. This happens even if I try to renew with the incorrect email address or the correct one
So, I've deleted the content of the /etc/lego/accounts/ directory as root but I'm still advised that: "It seems there is a valid certificate in the web server configuration folder. Please renew that certificate or generate new ones manually"
My /etc/lego/accounts directory is now empty so I'm not sure where to look for the 'valid certificate' which I intend to delete once I do find it...
Or, given where I am, what's the best way out of this so that I can start again?