Thank you so much for your help with this.
In my Mail anti-virus settings on Kaspersky, the "check IMAP" box is ticked.
And in "Threats and Exclusions" Microsoft Office is listed as a "Trusted Application".
I am delving into parts of my computer that I don't usually look at here!
As in that article,my gmail account is fine, it's just my ntlworld account that isn't.