OK folks I finally managed to sort it. When I set Thunderbird up, it didn't have a Sent or a Drafts folder, so I put them in manually. I went into 'Copies & Folders' and it said it was going to put copies into those folders. It all looked deceptively fine. What I didn't know was that I had to click on the 'Other' button and navigate my way to the Sent and Drafts folders I had created and tell it to put them in there. It works now. It took me four hours to figure that out.

re :When I set Thunderbird up, it didn't have a Sent or a Drafts folder,

If it is a POP mail then you would not have a 'Sent' nor 'Drafts' folder until you sent an email at which stage one should have been auto created.

However, if you are talking about an IMAP mail account then you would need to subscribe to see those folders then set up account Settings to use them. Creating additional ones may cause confusion. If account Settings was already set up to use them, but you had not subscribed to see them, then TB would have an issue. IMAP: To subscribe tosee a server folder: Right click on mail account in Folder Pane and select 'Subscribe'. Select/highlight folder (s) and click on 'Subscribe', then click on 'OK'. Note: only imap accounts can subscribe to see folders on server.

Sent mail (using IMAP) not saved in sent foloder. Message - 'Message sent but copy not placed in folder due to network or file access errors' I believe all settings are correct.