We discovered that your husband had been spending a large number of pound notes all over...

Mark Halliday:

When did you find out, sir? Chief Insp. Hubbard:
Well, the first clue came quite by accident. [to Margot] Chief Insp. Hubbard:
We discovered that your husband had been spending a large number of pound notes all over the place - it ran into over three hundred pounds - and it appeared to have started at about the time you were arrested. Now, I had to find out where he got that money, and how. Then I remembered that after you were arrested we searched this flat, and I saw a copy of his bank statement in that desk. So yesterday afternoon, I went to the prison and asked to see your handbag. While I was doing this, I managed to lift your latchkey. Highly irregular, of course, but my blood was up. And then this morning when your husband was out, I came back here to look at his statement. I never saw it... because I never got through that door. You see, the key that I'd taken from your handbag didn't fit the lock!