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Thanks, neveracrossword. Unfortunately we had gale-force winds yesterday & they blew down our main gate. (As it weighs over 300kgs, this is some feat, but involves blowing it off the end of the rail.) It took all day to muster enough help, but it's now back & chained in place.
As a result, I am only now catching up with it & will short-circuit the process by using mouseymick's very helpful details.