The Hold Up - S10-E3
Continuity mistake: When the robbers have nodded off and Mr. Spooner and Cpt. Peacock cut themselves out of the rope tying them to the cabinet, a clock outside is sounding the hour; if one listens it chimes nine times. But minutes later, after the rest of the group have come in and they make the call to Mr. Rumbold, the latter exclaims "It's after midnight." upon answering the phone even though less than ten minutes have passed.
The Hold Up - S10-E3
Plot hole: When the robbers come onto the floor and go into Mr. Rumbold's office, the staff suggests calling the police, only to be told by Mr. Harman that the only outside line is in Rumbold's office. This, despite the fact that there have been numerous occasions throughout the series where characters made and received personal calls on the department phones (Mrs. Slocombe calling her cat, Mr. Humphries calling his mother, etc).
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.