She's carrying my child but now refuses to talk to me?

Well I'll give you the short version. Basically I fell in love with this girl, we moved in, got engaged, had unprotected sex a lot and then well she started getting very, well bitchy and threatening. Then when I go up there to patch things up because I felt bad about leaving her the way I did, she tells me she is pregnant by me. Now since then a lot has happened, I helped her move, picked up her friend from burnley (baring in mind I live in Essex), we then don't see much of each other for a while and then we do and then well she refuse now to talk to me. I have said that I want to sort things out on a purely platonic level for the sake of the child but she just doesn't want to know, saying you'll never know if the baby is yours or not and all you cause is problems, the last part isn't exactly true and she knows it otherwise she wouldn't of stuck by me for 18 months. What I want to know is assuming she is pregnant by me, what can I can do to assert my rights legally as the babys father? Can I get any balls set in motion now? Yes I have tried talking to her and she just refuses point blank so that's not really an option right now. Or do I cut contact with this girl knowing that potentially she is going to give birth to my child one day and I will be excluded out of that childs life.