Email Problem

we'd like to change the password that was foist upon us when all the extra security malarkey was added last year,

You cannot change the email password by logging into the email account itself. To do that you need to log into the "My Virgin Media Account" (MVM)  for that email address.

If by "extra security malarkey " you mean that you have changed the username of the account to a non VM email address (presumably an Outlook.com address as you mention in your post) .then you need to sign into the MVM 

https://www.virginmedia.com/my-virgin-media

                                          with that non VM email address and the current VM email account password. 

Once you are in the MVM account, select Account Settings > Select the Virgin Media Mail Settings option

VM will then send an email to your non VM email account and you will need to enter the verification code into the MVM screen.

Then scroll down to the App Password section and click the Generate New App Password button and follow the instructions.

Then use that new  App password supplied by VM in email settings in Outlook.

Coenoby

Tried every permutation ie the outlook email addy with the present ntlworld pass (and also the outlook pass), and also even tried the ntlworld addy with the ntlworld pass or the outlook pass. It either does absolutely nothing, without any explanation, or it tells me that email or password is incorrect - which is the same email and password I've just cut and pasted out of Thunderbird, which is working just fine in sending and receiving emails! None of the above gets me into the settings for the NTLworld email which is all I'm trying to do.

Have I tried signing into the MVM account? Yes, it's what I've been trying all along.

Now when you say "that" email address, are you talking about my email (the main account) or her email, the secondary account?

Yeah except I've repeatedly tried to log into MVM account using her outlook addy and then using her NTLWorld password (which these days is the four part app password Virgin gave us last year) and it just comes back with "Incorrect Email or Password" without actually pointing out which one it doesn't like. Either way it's not letting us into the account details. If you're saying the NTLWorld password (the four part hyphen one) is not the same thing as the MVM account password then how do I go about gaining an MVM password if it won't let me into the account to make one?