The conversation that she had overheard, although interesting in that it proved beyond doubt the complicity of both Rita and Boris, threw very little light on the present preoccupations.
Rita and Boris were working together, it was made clear by something that was overheard, but it didn't make the situation any clearer.
mnli hypothesis: Rita and Boris were working together, it was made clear by something that was overheard, but it didn't make the situation any clearer. premise: The conversation that she had overheard, although interesting in that it proved beyond doubt the complicity of both Rita and Boris, threw very little light on the present preoccupations.