The username and password for your default/1st mailbox is just your account username/password, but for your additional mailboxes it's in the format of username+mailbox and the password you set when you created the mailbox.
Once you're logged in, it's actually just Settings you'd click on as it doesn't look like there is a Personal Settings.
Sub question: When I was with TalkTalk as an old Pipex customer, I could set the emails to deleted from the server at a set time after I downloaded them. Is this possible with Plusnet, please?
It is possible but not directly through us, you'd do that through your mail client (like Outlook) when it collects your mail. You'll need to setup your emails as POP3 as opposed to IMAP. Bit more information about that Here.
I hope this helps.
If this post resolved your issue please click the 'This fixed my problem' button. From 31st October 2022, I'll no longer have a regular presence here as I'm moving on to a new role within Plusnet.