Herbert Lom’s face was always suited to comedy, and in this film he really shines alongside Cecil Parker—someone who never received enough recognition but consistently brought a reliable level of comedic awkwardness to his roles. Guinness and Sellers were performing as expected, so their contributions didn't particularly strike me. Katie Johnson receives considerable praise for her portrayal of "Mrs Wilberforce," although I’m not sure how much acting was involved; from what I’ve seen in interviews, she seems to have largely played herself in this somewhat darker Ealing comedy. The story revolves around Guinness and his accomplices posing as amateur musicians in her dilapidated home while secretly plotting a robbery that could yield them £60,000. What unfolds is a skillful combination of close calls and carefully timed jokes that occasionally create genuine suspense; however, I don’t consider it to be as strong as the studio's work from the late 1940s. It loses some of its appeal and humor in an attempt to establish a more unsettling atmosphere during the final thirty minutes, which isn't something I entirely appreciate. Despite that, it remains a classic film, and rightly so, especially now with its recent 4K restoration, which really makes the colors pop.

Source on TMDB