As soon as I try to insert a new line it jumps out of the task so I cannot edit existing information
Could you please clarify for me how you are accessing Todoist, though the web-app, the desktop app for windows, outlook?
How are you trying to enter a new line? Ctrl+M will insert a new line into the task name.
Thanks - that works, but where is it documented. I have looked at the help information listed and it does not appear anywhere!
and I am using the web app to start with
This is not documented because it's a deprecated feature. We no longer support it and we can't guarantee correct functionality of the mobile apps if you use this method in tasks.
If you want to add long text into a task, please upgrade to Todoist Premium and you'll be able to attach notes to tasks which can be as long as you want.