A warm welcome to the forums. For total clarity, are you referring to @account.plus.com email addresses or one of the multitude of legacy vISP mail addresses?
Are you saying that you cannot log into the user portal with your account-name and password?
Is this an active Plusnet (paid for) account or just a legacy mail only account?
What happens of you send an email to firstname.lastname@example.org from Gmail - do you get a Non-Delivery-Report (NDR)? The receipt on a NDR might indicate a fundamental issue with the account.
The fact that you cannot log in via webmail (using account-name or account-name+dad) somewhat eliminates the likelihood that Outlook is encountering an network issue which is 'presents' as being a password error.