Where Does This Phrase Come From?

[Cloverjo]

Every time someone answers the current 'Now we know' thread I hear that phrase said in a slightly sinister way coming from a tv programme. I can't for the life of me think which one, and it's driving me slightly mad. Can anyone help?
I'm thinking League of Gentlemen, or at least one of the actors from that in something else.

[Mamyalynne]

?

[Cloverjo]

Of course! Thank you, mamya :)

[Mamyalynne]

You're welcome.

[Old_Geezer]

And now you know.