Question: If Mrs. Slocombe had squatters in her new flat, why didn't she simply call the police and have them removed?
Question: How is Catch-as-catch-can played?
Answer: Catch-as-catch-can is a style of wrestling in which the wrestlers are allowed to use methods which are illegal in most traditional wrestling matches. These include tackling, tripping, and holds below the waist. The humor lies in how the character of Mr. Humphries references it in his line: "I was known as Hugger Humphries. They had to pull me off people. I was the 'catch as catch can' champion of the Barley Road Mixed Infants."
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.