New Mac mail does not go to inbox but to a folder 'on my mac' (IMAP account).

Yes! A long time ago which I had totally forgotten about. Hadn't used the account for a while and the old folder had been deleted. When mail was received a new version of the folder was recreated automatically (I wondered why it kept doing this when I kept deleting it.)

Fixed now.

Should have thought of this.

Thanks for your help.