The Kipper and the Corpse (a.k.a. Death) - S2-E4
Revealing mistake: In the scene where Basil and Manuel have put the corpse in the room behind the reception, and the Major comes over and speaks to Basil, the actor playing the corpse blinks.
The Kipper and the Corpse (a.k.a. Death) - S2-E4
Revealing mistake: Just after Polly comes down to to the kitchen to tell Sybil that Mr Leeman's dead and they leave, Basil wanders back in, to learn the news. Watch the wall on the right on the staircase when Basil shoves past Sybil on the stairs. The wall shakes so much at that point it's virtually impossible to miss.
The Kipper and the Corpse (a.k.a. Death) - S2-E4
Revealing mistake: When Basil, Manuel and Polly are trying to hide Mr. Leeman from the other guests, they put him in a cupboard. While they are talking to the people staying in the room, Mr. Leeman's hand comes out of the cupboard. Watch carefully, and you can see the actor opening the door from the inside, and then putting his arm down, making him look like the corpse.
The Kipper and the Corpse (a.k.a. Death) - S2-E4
Revealing mistake: The dog's mouth is nowhere near Manuel when he is "bit".
The Kipper and the Corpse (a.k.a. Death) - S2-E4
Revealing mistake: When the characters are in Mr Lehman's room discussing his death, the backdrop outside the window has clear wrinkles and fold marks in it even though it's supposed to look like the outside.
The Kipper and the Corpse (a.k.a. Death) - S2-E4
Revealing mistake: When the sausages burn the first time, the smoke comes from off screen.
Answer: It's "Leisure Complex" by Dave Gold and Gordon Rees. Can be found on YouTube. The relevant clip is around 25 seconds in.