Other mistake: After Mr. Tebbs finishes telling his story about selling seven toilets in one day, Mr. Lucas makes a wisecrack and Mr. Humphries says "I think it's the way Mr. Tebb [instead of 'Tebbs'] tells it."
Continuity mistake: Normally the windows in Mr Rumbold's office are latch swing open style, but when the staff are viewing the parade they are plainer push up windows.
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.