Well, they've given us four possibilities. Minty and Billy wouldn't make for very good storylines, Billy has nobody and would probably be chuffed and she was married to Minty so nobody would really bat an eyelid.
Sooo, it's either Phil or Darren (if it's any of the people they lined up for us... I can see them tricking us!) because of the drama it would create. Phil because of how Shirley feels for him and Darren because it would be completely hilarious and, as somebody has said, something has to go wrong for him now that he's engaged.
Darren's already got enough storylines going on, what with his engagement, libby's dad being back and libby's mum/lucas the murderer being back on the scene... that family have enough on their plate without adding this. It would just be a big complication and they tend to keep things quite simple.
Phil doesn't have much going on, the Sam plotline is a bit thin on the ground at the minute giving him plenty of room for another story. But, why would Phil have sex with Hev? He wasn't drinking at the time and he'd have to have been... really. That said, Eastenders are constantly forgetting their storylines or getting details mixed up. One minute Hev's having a girl, the next they're all guessing the gender in her living room and then it's a boy and Patrick is saying it should be a girl... messy.
I'm thinking it's more likely to be Phil and they've messed up the timeline, because her rambling phonecall implied that the person might not remember it happening (which explains why not a one of the 4 look worried) or it's somebody completely different but I can't imagine who.
I clearly like this soap a bit too much...
and will smith said he wanted to have a part in it! 
