I am unsure of your issue. From your post, it seems that your wife's email uses the same domain as yours. If so, you could create an additional account on your Thunderbird. My confusion is that you also state she is using plusnet. She has two accounts?

to clarify, we were both with plusnet webmail but Plusnet set me up with thunderbird which I have been using for years, my wife has little emails so left it with plusnet, but recently plusnet are ceasing to provide any emails and will be switching everyone over to a new company. As I am happy with thunderbird and now think the best way forward is for my wife to also use thunderbird. I still have the email as set up originally with plusnet using julian@lionel33.... but remains the same but comes into my pc and I open up thunderbird, but my wife has helen@lionel33..... but has to go to webmail with plusnet and difficult to logon. If helen can be added to the thunderbird as a new addition that would be great.

You can set her up with an accout on your PC or hers. You can get the setup information by clicking 'account settings' on your pc and copying the information from the 'server settings' pane and also from the SMTP server settings pane (which you can get from the 'account settings' pane and scrolling down left side to 'Outgoing server SMTP' and copying that information. Then, using her email address you will have all the necessary information. Be sure to note whether you are using POP or IMAP so you can set hers up the same.