Thank you Euler, that works

I am trying to remove attachments from old messages in folders (to cut down size) - something I have
never had a problem with.

Now when I select an attachment and click on Delete nothing happens.

Mike, you can't delete an attachment while the message is inside a folder. Move the message to NMF, delete the attachment, and then return it to the folder.

If you have a large number of messages, maybe you should consider a general rule set to deal with this burden. I have a set of rules to save attachments from certain messages as they arrive. It can be easily changed to deletion.