Sorry to be dense, but I cannot see where that option is. When I click
on Account settings, I can see all my different email accounts,
through which I still need to receive incoming mail.

Dbl-clicking on one brings up:

User Info: Name and Email address - this has the old account details

Account type: Pop3
Incoming mail server:
Outgoing mail server: 127.0.0.1

Logon Information
That old account's username and password

The "require logon using SPA" box is not ticked.

Then clicking on More Settings gives me 4 tabs: General, Outgoing,
Connection and Advanced.

"General" has that old account's name, and the two boxes under Other
User Information are blank. I've tried typing the new name into the
Reply Email box but this doesn't cause replies to come from my new
account.

As with my new account, there are no boxes ticked under "Outgoing
Server".

Connection has only a check in the "connect via Lan" box.

Advanced has the same port and server settings as my new account.

Do you think you could post a screenshot of where the "From" field can
be found in Outlook 07?

Thanks again for helping me with this.

Jo