When he said to alfie 'don't do anything stupid' ?

He was echoing what he said to Alfie before.

Why should he be taking the mickey? He cares about Alfie and knows he is not a bad person, that he feels guilty and never meant to harm Kat. He just had a serious conversation with him.

I thought alfie was going to turn round to him and say 'I already have'.

more to the point, Mick is very forgiving of Alife, then again he was similar with Ian, not sure I would be so forgiving.