Oh for gods sake. Them not knowing if the door was open or locked is easily explainable by them just not being attentive, something they clearly weren't being. Factor in some drink and its easy for them to not know such a detail and claim it was because if it wasn't they looks bad but then change their story when it was shown it was. Were they bad parents that night and any other night they left their children unattended, yes. Does that mean they are killers? get real. I suppose the MET are complicit as well? They've spent years investigating this but are her parents their prime suspects? They let their child down, something they will have to live with forever.