This really is sub-par for Hammer. I found this one to be hugely generic and quite a slog with poorly handled plotting and no real drive or direction.
Anton Diffring is good as the 104 year old Benner but the film manages to entirely miss portraying any menace. His need to keep statues of models he has killed is never explained even though this rouses suspicion from the Police. Similarly the insane woman in a cell is never explained. Overall a poor Hammer with none of the flair and style that its contemporaries were giving at the time.
Not a classic, avoid.