This has worked, thank you Allan. I've sent and received test messages to and from various addresses, all of which work, and everything is downloaded from the external ISP server.
I've ended up with two folder structures in Outlook, one headed 'Outlook' from the imported pst file, and one...
New Lenovo laptop running Windows 11, office 365 with Outlook downloaded.
On previous laptop (Lenovo/Windows 10, Office 2015 (I think)), 2 email accounts running in Outlook, thisname@externalISP.co.uk and thatname@externalISP.co.uk, both addresses would download messages to outlook from the ISP...