A Change Is as Good as a Rest - S5-E2
Visible crew/equipment: In the opening scene in the board room when Mr. Lucas is commenting on how Mrs. Slocombe is "big in the south," you can see the shadow of a boom mike floating back and forth across the table.
Answer: That wasn't possible in 1972. Until recently squatting was a civil offence in the UK, not a criminal one, and removing squatters was a complex and time consuming business.