there is a setting for paragraph mode at tools>setting>composition. Unticking that may address your issue, but I have also seen this happen, depending on addons.

Hi David Thanks for getting back to me with your suggestion, but when composing an email, the tools menu does not appear! Do you know why that might be? Please see image below. Regards Brian

You're saying it shows 'tools' but when you click, nothing happens? Strange. I suggest to do this from main menu to set the option.