Configuring preferences and options in Outlook.com
As an interim solution I moved the account across to Fastmail where I have an account for family email and guess what, it worked and hasn't stopped working yet as not having access to email was causing problems.
I'm going to guess Microsoft Outlook Support don't feel loosing emails is important enough to warrant a reply to the ticket raised highlighting it as an issue.
I hope they solve the problem so I can use the Office 365 account as I'd like to, you know as advertised.....