You panic too much, you would be doing all this from inside Gmail so if it doesn't work it just means you enter a setting wrong, it won't harm your other email accounts.
Just try it rather than talking about it.
---------- Post added at 20:28 ---------- Previous post was at 19:57 ----------
Just tried importing from ntlworld.com to my gmail.
In Gmail click the cog in the right corner
Settings
Accounts and Import
Import mail and contacts
Put in the email you wish to import from
Email password
I left the pop3 settings alone, then clicked continue and you get a few choices on what you wish to do, with check boxes.
import mail
leave a copy of retrieved message on the server
archive incoming messages(skip the inbox)
add label to all imported emails(from email)
Then click start and it says it can take several hours to 2 days.