Unfortunately, it's not a simple step. I wish it were. This is the official (to my knowledge) documentation. Well worth the read https://support.mozilla.org/en-US/kb/message-threading-thunderbird

Here is my own documentation on threading, duplicating the above, but it may help: You need to highlight each folder and click View>sort by>unthreaded. If this applies to just a few folders, just repeat the process as necessary. If you have many folders or plan to add many new folders, you may wish a different approach, below:

More info: 
To change default for *new* folders, In config editor, you can change default that applies to NEW folders you set up:
mailnews.default_view_flags      0 =unthreaded  

Config editor is available at Tools>settings>general and scroll to bottom right. 
  ------------
	
To set many *existing* folders, follow these steps:
This must be done on an account by account basis.
- select table view  - the icon just to right of 'quick filter' button above message list pane
- select a message folder and apply all desired attributes - View>sort by from menu bar (if not shown, click alt key)
- click the icon at right side of column display in message list pane
- select the entry near bottom 'Apply current view to...'
- highlight 'folder and children'
- move mouse left to see account name and move left again to display all account folders. Now CLICK.
- a small menu will appear to confirm setting all folders within that account.

I did a simple (okay, crude) video at https://davidkirk.org/SettingDefaultFolderView.mp4


ALSO, for future installations of Thunderbird, 
- create a simple text file with name of user.js with these lines: 
     // Set default threading to UNthreaded  ###   <== this line is optional, just for documentation
     user_pref("mailnews.default_view_flags", 0);
- install Thunderbird and exit prior to setting up accounts	 
- copy the user.js file into the profile
- now, when you start Thunderbird, unthreading will be the default. Problem solved. 

Many thanks for your help David, I have carried out unthreading the long winded way and am now waiting to see if something triggers a reversal One thing I did notice my email is a gmail.com address, I looked and noticed that was threding so have changed that to unthread I will keep your advice for future reference, once again many thanks John

You're welcome. I agree, it's rather dicey to unthread everything and be sure to do a backup when everything stabilizes.