This is a better-than-average entry in the Saint series - It holds your interest and, as mysteries should, keeps you guessing until the end and has several suspects to choose from.
Many films from the Golden Age are not for all tastes, especially younger viewers. They date themselves by clothing, cars, settings, etc. Who nowadays asks for a highball? Or wears a suit and tie everywhere? And the legal process was so much simpler - must have been a dearth of lawyers back then. Frankly, much of value is missing from those days.
In any case, go with it and enjoy. It's good - in an old-fashioned sense.