Well this is progress:
Second question first, yes I did use a capital T for Trash.
See image below: which I hope will transmit.
First Question: Individual Deletion: Yes I can.
If I open the Trash folder and highlight one message then click the big red delete X at the top of the page the message is deleted.
Also If I open the Trash folder and highlight one message then right click the mouse I can select Delete from the dropdown menu and that also works.
So it appears that it’s the empty functionality that isn't working as you suggested above.
Thank you for sticking with this.
Its possible you may have to live with that limitation, it may be because WLM is now obsolete the way the empty functionality is implemented is perhaps not supported by BTMail. That is a pure guess, I don't have any knowledge that it is so.
It might be worth emptying the Trash folder of all the mails you previously moved into it and then seeing if new mails sent to Trash by deletion from the Inbox can be deleted by the empty function. Hope that makes sense.
Thank you for all of your help.
This is definitely an improvement on what I had, which was nothing.
In fact I have now discovered that if I open the Trash Folder and then select all of the messages, I can use either the Main Delete button at the top or the Delete in the dropdown box to clear them all in one go.
That is perfectly workable and I can live with that.
The reason I wanted to change all of the accounts to IMAP is that I am in the process of installing Outlook 365 (because WLM is getting old) and I want to run them in tandem until I fully understand Outlook 365 and discover how to transfer all of my sent & storage folders across (18 years’ worth).
With both WLM & Outlook 365 set up with IMAP, what I do in one email client is reflected in the other. If one is IMAP and the other is POP you have to do everything twice.
So once again thank you for all of your help and perseverance.
If I do ever discover how to fully fix it I will add a post below.