OK, some elements, such as the spy who was also a member of the England cricket team were a little far-fetched, but I swallowed it. I also - for the sake of giving it a go - the idea that Britain's (or as Blake has it, England's) secret state was overseen by well-meaning anti-fascist liberals.
No, what annoyed me was this scene, which is echoed in every Bond film/novel since
'Ah, so you are spying on me. I shall kill you. But not for another half hour because I'm a bit busy. You may as well read my secret plans to take over the country and invite the Nazis in. And I'll tell you the secret plan under the secret plan. Because you're going to die. In half an hour. Wait here. I'll just check you for hat-pins. But you can keep your lighter'.
Our heroine then sets light to her room, smashes the door down, hides behind the next door until the villains dash in, gets out, locks them in the burning wing of the mansion, and escapes, not forgetting to pick up her fur coat and commandeer a Rolls Royce. The villains survive too.