Mr Cargill

In the novel The Body in the Library, Mr Cargill is mentioned by Miss Marple. He is a builder, who bluffed a lot of people into having things done to their houses they never meant to do. Often it became very costly for them. However, he was always able to explain his bills away plausibly.

Miss Marple likened him to Mark Gaskell, as they both married into money.

BBC's Miss Marple
Mr Cargill is mentioned by Miss Marple in the 1984 adaptation.