Believe it or not, I had tried that, several times, and it kept resetting to the original value. I don't know what I did but after three or four goes just now, it successfully cleared.
However, this is NOT as good as the old detach template, as due to the new inheritance rules, it removes access to all the inherited code, rather than copying it into the detached component. As such, it makes it more complex to make something based on but divergent from the template behaviour.
So, much cut and pasting later, I am now (kind of) back to where I started and can remove the panel and popup menu, which is all I really wanted to do. 🙂