Joe's mum has been very sick plus there's been some family loss on their side over the past year or 2. As an adult, I'd imagine they've probably had a mature discussion about how important is for them BOTH to spend time with their parents. I know not everyone agrees, but my parents raised me to believe you have a life, your partner has a life and you have one together, perhaps they're similar. Having said that, if Diane really wants to be married and settle down with a baby, then she shouldn't have to sacrifice her dreams waiting around for another Alfie Deyes type, non-committal man child.
PS hurry up and do something interesting Joe so we have something to talk about