Thank you.
It seems all is not lost.
I have had Windows Live mail on my desktop since the days of XP. I really like it.
When I got the laptop (second hand) a couple of months ago it was never intended to be for my main use, just something to take away with me to keep up with things.
Windows didn`t seem to have an email programme I could find for the laptop so that`s why I downloaded Thunderbird.
It has totally emptied my BT mailbox and downloaded everything that was there. Inbox and deleted messages - the lot.
I ended up with hundreds of deleted emails which had stayed on BT.
I have spent quite a few hours on Thunderbird going through them all manually and deleting those I really don`t want.
Many were multiple notifications of replies from Rootchat
and emails from Ebay etc.
Then I started on Outlook on the desktop. Thankfully everything there was unaffected, I had folders for various people / subjects and they were still there. Some I had forgotten about. There are some from 2012 when I first started an interest in History.
I have been playing with both Thunderbird and Outlook and it seems that when an email comes to me through from BT it goes to both of them.
I deleted the unwanted ones permanently and they were removed from BT as well.
I think I need to keep a weekly check on them all. 
I also found all previous emails from quite a while ago that had been saved to a folder when I had my desktop updated to 10. The shop I took it to did that for me. I never was about to find them till recently. 
I now want to know how do I save emails to a folder myself on my hard drive or even a memory stick.
Thank again.
I have been using Outlook on a Windows desktop since at least 1999 (I ordered some CDs from Amazon in October of that year and still have the email) and have all my emails stored in a large folder structure; I tend to file or delete new emails once a week or have a daily automated backup system. New emails are downloaded to Outlook whenever I open it and removed from the mail server at the same time.
There is only one* file that needs backing up:
C:\Users\<Username>\Appdata\Local\Microsoft\Outlook\Outlook.pst
*Mine got too large so I created another, so in my casee are two files.
The pst file (or files) can be copied into the same folder on another machine and will immediately work with a local Outlook installation there; I do this when I travel with a laptop and then just copy them back to my desktop when I return, replacing the old ones.
That system has served me well and I see no reason to change it.