What Formby actually chose was something that gave him the tone quality of a banjo (well established in jazz and dance bands by the time he started out) but volume and range better suited to his voice and material.

A banjolele, in other words - a uke with a banjo drumhead, or a four-string banjo tuned like a uke. Or so I've heard (I don't play anything with strings myself, so I may be burbling).