Three Witnesses

Someone gets murdered but it's very nonchalantly handled in this slick but talky and meandering Twickenham Studios potboiler complicated - as Leofwine_draca has already pointed out - by Henry Kendall and Sebastian Shaw looking more like each other than Shaw and the actor supposedly playing his brother. (Kendall had just played a double role in 'Death on the Set' and they could have saved the trouble of trick photography by simply casting these two.)

Fortunately, figuring out who was behind it all is simple for anyone familiar with George Formby's films of a few years later...