I was the first officer on the early morning departure MAN- LHR in a BA Super 1-11 when after take off we had a nose wheel green. We returned to MAN and having had a quick inspection by an engineer took the aircraft back to the gate. I still occupied the flight deck when an engineer appeared carrying the nose wheel ground lock which had caused the problem. It should have been stowed in the u/c ground locks box to the left of the captain's seat. With a grin and a "how stupid pilots are" expression on his face which quickly disappeared when he opened the box to find one already in its correct place. I'm sure that would have never been mentioned had I not still been there.
One of the pre departure checks was "ground locks stowed" and only one set was supposed to be used on each BA aircraft but obviously not in this case!