I really appreciate your patience :-) I realise I'm shredding your nerves...
Having had some sleep and time to mull this over I just want to summarise this unfortunate thread in case it helps any other newbs:
Starting from the top again:
If my server is called sierra.qwerty.com, please help me with the following:
The Roundcube installer asks:
The IMAP host(s) chosen to perform the log-in?
To use SSL/IMAPS connection, type ssl://hostname
But the Roundcube installer will give you a "fail" when you test it if you don't have an appropriate DNS record pointing at the server.
If the name of your server is "sierra.qwerty.com", then the hostname for your mailserver is *always* "sierra.qwerty.com", for any protocol. Period. Assuming everything is running on the same box.
The difference is in the ports the mail services listen to.
If you look here:
you will see that the mail ports are:
POP3 - port 110
IMAP - port 143
SMTP - port 25
HTTP - port 80
Secure SMTP (SSMTP) - port 465
Secure IMAP (IMAP4-SSL) - port 585
IMAP4 over SSL (IMAPS) - port 993
Secure POP3 (SSL-POP) - port 995
Use the appropriate ports for the protocol and be sure that the Firewall has those ports open.
The reason you may have been getting confused is that when you use your ISP's email server, you often see something like "imap.myisp.com" or "pop3.myisp.com"
This simply means that they have dedicated machines for those protocols. ie, imap is running on a machine *called* "imap.myisp.com" etc.
Please tell me that's right so I can have another go at my install