it will (or should ) be Derek imo. It can only make sense for the shagger not to be visible in the story, if the story isn't really about him at all. Derek is imo the only choice that would make the lack of the mystery man's involvement OK.; he wouldn't give a stuff about Kat or Alfie, and would happily stand back and laugh as their marriage falls apart. I actually don't understand why people try and reason a possible 'culprit' due to it having more depth or something. Trying to argue in retrospect that the affair had so many layers to it would be a complete cop-out, as we wouldn't have seen any of it. Im just going with the outsider because I don't want it to be Max again and the writers are making it obvious with Derek now, just like they did firstly with Ray then Michael, both have now been ruled out lol. Im sure Roxy and Amy were living with Jack for a bit when Phil kicked them out (may have got that bit wrong) x