Revealing mistake: When Captain Peacock goes behind the curtain to the hall way to Mr Rumbold's office, you can see that the sets are right next to each other and the other actors are waiting in there for the next scene.
Revealing mistake: When Mr. Lucas gives Mrs. Slocombe the radio mike, it slips down her dress out of reach; Mr. Harman says "It can't have gone down all that far," and he's right: The short cord of the mike is visible sticking out of the dress for quite some time until Mrs. Slocombe herself pushes the mike down (in the appearance of trying to fish it out).
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.