I find it hard to remember all the different PLaNC permutations but from what I can recall...
Nick & Leanne were married as teenagers. Can't remember why they split up.
Leanne married Peter. At some point prior to their wedding I think she had an affair with Nick. Leanne and Peter divorced because he had an affair with Carla.
Nick and Leanne got married again I think. He slept with Kyle just before the wedding and then had a brain injury when David attacked him. I think that led to him divorcing Leanne again but I can't remember if they actually got married the 2nd time
Peter and Carla got married. They divorced because of his affair with Tina.
Carla and Nick got married. They divorced because she slept with Robert. I think the marriage lasted about an hour.
Nick and Leanne got back together again. They split up because he was jealous of all her baby fathers (Peter and Steve). He almost died in quicksand but Peter unfortunately saved him and he left the Street. But sadly not for long...
It now seems Peter and Carla are getting back together.
It now seems Nick and Leanne are getting back together.
I'm sure I've missed out a fair bit