Communication Problems (a.k.a. Theft) - S2-E1
Plot hole: Basil torments Mrs. Richards by miming when he is speaking to her, so she will think her hearing aid has failed. However, she speaks twice during the scene, and she would hear herself - obviously - and would know that he was trying to fool her.
Chosen answer: Indeed it is Polly. She has stuffings in her mouth to pretend she is swollen.
Mortug