select: 'View' > 'Threads' > 'All'

The 'Display message threads' icon - usually the first column header - when this is selected/clicked on, it will show emails in threaded format. If you have threading selected: Click on that header icon to remove threading in that folder.

You could then remove that column header to help prevent accidental reselection. click on far right header icon - 'Select columns to display' and select the 'Threads' option to untick/deselect and that column header will be removed.

If you have threads enabled but need to only see the unread emails in that folder: In the Quick Filter toolbar click on 'Show only unread messages' icon - looks like a pair of sunglasses. click on that icon again to go back to original view.

Sorry, but this didn't work, I painstakingly went through every single folder I have, and removed the threading, low and behold, 2 months later, my sent box is showing threads again- I need a permanent fix, even if I have to alter code, could you please point me in a way forward, thanks for your help.

Are you using CCleaner or similar? This destroys valuable settings files.

I use View|Folders|Unread quite a lot to check for unread messages, particularly as some are moved automatically to subfolders by my filters, so switching between threaded and unthreaded would be pretty irrelevant anyway; my unread messages are distributed across multiple folders.