I can't see how anyone can know. Like KitKat says, Tamwar would be discreet as would Denise. And it's true, Shirley was shocked when she found out Sharon knew.
When Shirley drunkenly mentioned it in the pub, there was no one in there who is a regular character to find out.
If it was public knowledge, I would imagine there would be some gossip going on or at least scenes where it was spoken about.
I think it must just be Denise, Sharon, Phil, Tamwar and close family who know. I imagine Elaine does? I wish we had seen Denise find out on screen and seen her reaction.