This may help you achieve what you want https://support.mozilla.org/en-US/kb/how-create-and-use-mailing-lists-thunderbird

Contacts can be copied between address by drag and drop, while holding the Ctrl key. Otherwise, drag and drop moves, not copies.

Thank you both for your help. I have now managed to produce the list I require by following the link suggested. I had tried to drag and drop by trying to have both lists open at the same time. Learning that I only need to drag to the list name has been a useful education