Can I get Office 365 (including Outlook classic) on ARM64 now?

Thanks I will have a try later. (Going out now for some hours)

Sorry I was busy daytime today. I have now tried the route you suggested, beginning by Uninstall on the 32-bit version of Office which is what I had this morning. The Uninstall was not complete: the "uninstall" troubleshooter told me "there is a problem". I could remove Office 365 with Control Panel - Programs but there was a lot left, including all the files in Office 16.
The only route offered for manual Uninstall looked very complicated, so I thought I would try to see whether I could just install over the top. The regular default installer appeared to work, but WORD.EXE EXCEL.EXE AND OUTLOOK.EXE are still the versions I installed yesterday, so probably 32-bit. (I now wonder if the installer picked up that I had a 32-bit version already and downloaded a 32-bit Officesetup.exe? It's not quite the same size as the one I got yesterday).
I still have OUTLOOK.EXE but it now gives me an error message when I try to open it: though it did bring up a Window.
Now feeling that I should probably go back to either the System Restore point which I created today before I started, which was an apparently-working 32-bit Office 365 including Outlook.exe, or perhaps one I created on Sunday night which was before I started doing things with Office. I think Word and Excel were working at that point, and presumably 64-bit, so I can try getting Outlook.exe from MS Store. Please say if you have any comments on what is most likely to work without excessive effort.

Thanks for your answer !

Completely uninstall Microsoft 365 by following this tutorial : https://support.microsoft.com/en-us/office/uninstall-microsoft-365-or-office-from-a-pc-9dd49b83-264a-477a-8fcc-2fdf5dbf61d8

The troubleshooter should remove any leftover residues. Once you've completely uninstalled Office, try again with an install from https://account.microsoft.com/services. The Microsoft website should give you the latest ARM64 version.
When you're downloading the installer, make sure you've selected the 64-bit version (see screenshot) :

OK... the link https://support.microsoft.com/en-us/office/uninstall-microsoft-365-or-office-from-a-pc-9dd49b83-264a-477a-8fcc-2fdf5dbf61d8is the same as the one I had tried to follow before. It did not work yesterday, and it did not work today - the contents of Office16 were left untouched. However I realised that I was running in a non-admin user (which works fine for most software) & that another user was logged on. When I ran the uninstall troubleshooter in an admin user with no other user logged on, it worked (it still gave an error message but everything had in fact gone), and the verification confirmed that. Then I could install Office 365 64-bit from the link that you gave above, and the installation even included OUTLOOK.EXE. I haven't finished connecting it to my existing pst file but things do look OK. If you have any connection with Microsoft then I would give the following feedback:

1. Leaving https://support.microsoft.com/en-gb/office/choose-between-the-64-bit-or-32-bit-version-of-office-2dee7807-8f95-4d0c-b5fe-6c6f49b8d261 online with that now-incorrect advice to use 32-bit Office on ARM64 is unhelpful: this is not now recommended.
2. My original problem was that I bought a PC (an Asus) with most of Office 365 pre-installed, when I tried to find a way to get Outlook Classic it seemed quite difficult (the 1st problem was to find out that is the new name of Outlook.exe, and that it wasn't on my PC), and when I did find my way to Officesetup.exe, and ran it, it just sat there, doing nothing but with no error message. That was unhelpful. Perhaps the advice to use the MS Store version of Outlook Classic might have been useful - this time round I didn't need to do that.
3. There should be advice to run the uninstall troubleshooter in an admin user and/or with no other logged-on users.

Thanks for coming back to me !

Good to hear that your issue is solved.
This forum is composed of users like you and me. If you want to leave a feedback from Microsoft, feel free to use the feedback portal : https://feedbackportal.microsoft.com/feedback

If you ever find yourself in the same situation, you can download Outlook Classic from the Microsoft Store. Also, generally, all troubleshooting tools must be run as an administrator to be effective. This is the case for Microsoft tools.