Leanne was pushed over in an arguement at the flower van by Kyle (the guy arrested for kidnapping the daughter) who was selling smuggled fags from inside the van. She was unconscious, he took her to her Grandads caravan and tied her up. When she came round she panicked about her kids so Kyle went out to look for them. The son got away when the dog bit Kyle but the daughter ended up in the caravan with her mother. The husbands old army buddy was the mastermind behind the fag smuggling and he and Kyle dumped Leannes body in the lake thinking she was dead after she had lost consciousness again. Kyle had befriended the daughter who promised not to say anything.
Leannes dad overdosed because he couldn't get the image of the five week old corpse of his daughters body out of his mind and possibly he thought his wife may suspect he had something to do with the murder.
The baby was her husbands but she hadn't told him about it because she thought he was having an affair with the gym receptionist.