If the message is loaded from the server, including the body, can you not have it removed from the server automatically?

Moving it to local folders removes is from the inbox on the server but it still appears in All Mail on the server as an unread item. If I mark it read then move it it disappears from All Mail on the server but I cannot then mark it as unread - the filter rules has that greyed out.

sounds to me like your going a long way round to implement pop using an IMAP account. Why not just use POP and let it delete the mail as it is downloaded?

There is an option on the GMail site you have to set to allow the deletion request sent from Thunderbird to actually work. But it sounds like a lot less effort.

Note the mail is probably appearing in the all mail because it in in the deleted mail folder after the move or something silly. Gmail really don't like you deleting your mail.

I think finitarry has made me realise where I'm going wrong: I think I need to COPY the message to my local files and then DELETE it on the server! Need to test that...

Nope that deletes it from both places!

As every filter is within its account-container (?) there isn't a way to set email unread from that account if its in the Local-"account"

Maybe this would do the trick In account-folder

name="Status is: new" enabled="yes" type="17" action="Change priority" actionValue="Highest" action="Mark read" action="Move to folder" actionValue="mailbox://nobody@Local%20Folders/temp" condition="AND (status,is,new)"

and in local folders

name="prio-1" enabled="yes" type="48" action="Mark unread" action="Change priority" actionValue="Normal" condition="AND (priority,is,Highest)"