Right then. Lets cut through the nonsense.
Cory.
If you are the sole named person on the bills, the you are the sole person legally responsible for paying them. End of.
I take it you have a typical arrangement with your flat mates, in so much that, you "pay" for some bills, they for others, and you each pay back your equal share.
Unless you have this in writing, you haven't got a leg to stand on if someone decides they don't want to pay you back.
The boyfriend would fall into this category.
Your only option is to get the girlfriend to pay his share, or get him to.
Jonathan.