Deliberate mistake: In nearly all the episodes that the front door of the hotel is open we see a painted board that is meant to be bushes or something outside. Yet whenever we see scenes from the outside of the hotel there is no sign of any trees or bushes.
Revealing mistake: Whenever somebody closes a door, leans against a wall, or goes up/down the stairs and brushes against the wall, it wobbles, revealing that the walls are lightweight stand-up props.
Answer: It's "Leisure Complex" by Dave Gold and Gordon Rees. Can be found on YouTube. The relevant clip is around 25 seconds in.