In the 1930s and 40s, B-mystery films were very, very common. There also tended to be some cliches in most of them, such as having a cop (or cops) who is a total idiot and a guy who announces he's going to tell the police everything...and you KNOW he'll soon be dead before he can do so!! This picture has both of these but still manages to be entertaining and novel on occasion.
The most novel thing about the story is the initial crime. Someone has stolen state secrets...specifically, Britain's budget before it is officially released. Why could this be a problem? Well, someone knowing about this spending could capitalize on it in the stock market. But when this doesn't happen, you know that there's a bit more afoot.
The Inspector is played well by Gordon Harker and he is assisted with an absolute idiot, Sergeant Bingham (Alastair Sim). Again and again, the Sergeant reveals his stupidity and Hornleigh solves the case with just about no help whatsoever from Bingham. Per the genre, Bingham is the comic relief cop.
Is it worth seeing? Yes. While far from a must-see, it's a pleasant little crime picture. And, if you like this, the studio made several other Hornleigh pictures....though I have yet to see any of these.