OK let me try it out tomorrow on my OH's computer as he is still using TB! v1.63 unlike me who has moved on to v3... I'll be back tomorrow. 
OK thanks. It's not too urgent. I have a work-around for the time being.
It's funny because I have another similar filter that is searching for a sender's address, doing the filter correctly and then marking the message as read.
But this one is searching for a text string [FINALLY moving] in the body and should then be marked as read.
It's going into the Trash can as it should, but I don't know if it's because another filter is catching it later (down the list of filters). I don't think so, because that filter works properly and marks the messages as read.
I hope this makes sense.
Maybe I don't have the text string set up properly? At first I just had 'FINALLY moving' with nothing else. Then I added brackets [] around it because the Help file gives examples like that.