Found it ....... but it's different to how you remembered it. It was heroin, not cocaine. It wasn't Sheila who was contacted it was her friend Caroline and it wasn't in the party bags, it went missing from a drawer where the children's party had been held. Seems a bit odd that no one else had been contacted and Caroline doesn't mention the friends name. It could just be an embellishment. Either way, here it is.
This is from Murder At White House Farm by Claire Powell (Page 60 Chapter 4).
This is a recollection from a friend of Sheila's called Caroline (ironically).
"One of the girls had a boyfriend who was the 'Mr Big' of the drug-dealing world in central London. One day they threw a party for their daughter. my kids went and Sheila;s boys went to it. After my kids got home, my girlfriend rang in a panic, asking whether the children had brought home a bag that wasn't theirs. I looked and couldn't find anything. Then she told me that a bag of heroin had gone missing. It had been in an open drawer in the room the children were in. I was outraged. I mean, the kids could have just licked the bag and they would have been dead. I couldn't believe that the woman didn't realise what she had done. She couldn't understand the fuss I was making. I fell out with her over that.