I just wrote a long comment on Valentine's page about this. I take the view that books are canon and the tv series isn't (where it contradicts the books) and in the books there is basically a fifty fifty chance. In truth, I just don't know whether Valentine is George or Ross' baby, but, given most of the fandom seem to say he's Ross' baby, I'm inclined to say George for balance

This is what I said in my post:

It says throughout the article that Valentine's biological is Ross, but is this ever actually proven? Yes, he looks a lot more like Ross than George, but that could be because of Elizabeth. And, whilst it is unlikely that she would go into labour after 8 months with nothing happening, it's not impossible. Also, Elizabeth may still have fallen. We don't see her between her dropping the vase and being on the floor, she could have fallen. Also, like Harry Potter, I take the view that the books are canon and the tv series is not canon. In the books, it's told from George's perspective and there is nothing to suggest Elizabeth didn't fall down the stairs and that sent her into labour a month early. It seems like the tv people want us to believe that he is Ross' baby, but it's possible that the author thinks it's George's baby