lol thanks.
I'm getting good at it now having done it a few times mucking about.
In case anyone needs help, I use an punch to pry it into place and push the punch key through. Then push the punch out with the slave pin (hammer pin retaining pin) locking the slave pin into place.
Place trigger in place in the gun then knock out slave pin with the trigger pin.
Makes a nice little standard minor gun until my major gun is ready.