At 40m 18s this is Pinewood Studios. The building on the left has since been replaced with a real "Theatre Seven".
On the right is at least the third appearance of the giant white hand prop in this film.
This was Grant Taylor's (General Henderson) final acting performance due to being very ill with cancer (appearing quite emaciated), passing away the following year at age 54.
Only two of the alien spacesuits were made, so at no point in the series are more than two of the aliens seen on screen at any one time. These costumes were made of red spandex and, at the start of production, they were ornamented with brass chain mesh, as seen in the episode Survival (1970) (fourth episode filmed). Later, this was replaced by silvery panels. The dark vertical bands on the sides of the helmets were slits meant to allow the actors to breathe.
During the opening credits, the U.F.O. theme music is modified with some jazz and blues elements along with additional tympany.
The episode featured scenes from two previous episodes: Identified (1970) and A Question of Priorities (1970).