Goodbye Mrs. Slocombe - S10-E1
Other mistake: When Mr. Humphries first comes back to the department in drag and talks to Capt. Peacock before being directed to the Ladies Dept., you can see, in the background, Mr. Spooner trying desperately (and unsuccessfully) not to laugh.
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.