I'm afraid it was outlook.com as that is the only way I could find to import address book entries. .CSV file from (Office) Outlook into Outlook.com and then link that account in Win 10 Mail. As I said - messy.
Thanks for your comments, I was afraid those might be the answers.
The problem is that new computers only seem to come with Win 10 nowadays so we are just going to have to get used to it. I think I'll stick with my old Win 7 machine!