With the January transfer window now open, Sky Sports pundit Paul Merson casts his eye over where the various teams vying for the Premier League title must strengthen in order to maintain their title challenge. Just 12 points currently separate the top eight sides in the table, making this one of the most exciting and competitive championship races in the Premier League era.And that means a key new addition this month could very well prove to be the difference between winning silverware and not, or finishing in the Champions League positions and outside the top four. So who does The Magic Man think the title contenders need to snap up before the winter window slams shut for business at the start of February…?ArsenalI think they need a defensive midfield player, if I am being honest. They missed the boat with Morgan Schneiderlin in the summer, but someone like Victor Wanyama – not great on the ball, but if you can get him to win the ball and give it to someone who is better, then that is someone I think they need.I can see the Troy Deeney one happening, I think he is a good footballer and works very hard…