Ok, the auto-filled details for the server name were incorrect. I had to adjust them to the following (highlighted in Bold);
IMAP information for the incoming iCloud Mail server
Server name: imap.mail.me.com
SSL Required: Yes
If you see an error message when using SSL, try using TLS instead.
Port: 993
Username: this is usually the name of your iCloud Mail email address (e.g. johnappleseed, not johnappleseed@icloud.com). If your email client app can't connect to iCloud Mail using just the name of your email address, try using the full address.
Password: generate an app-specific password.
SMTP information for the outgoing iCloud Mail server
Server name: smtp.mail.me.com
SSL Required: Yes
If you see an error message when using SSL, try using TLS or STARTTLS instead.
Port: 587
SMTP Authentication Required: Yes
Username: your full iCloud Mail email address (e.g. johnappleseed@icloud.com, not johnappleseed)
Password: use the app-specific password that you generated when you set up the incoming mail server.