Thanks to those who replied; I've tried to post this about 3 times now with no luck so getting a bit paranoid.
I've always had a niggle about how Cat treated Jon; I know it can't be easy seeing physical evidence of your husband's infidelity, but with the way they lived, she could have as much or as little to do with his upbringing as she wanted. It's not like they're stuck in a 3 bed semi. . . .
I know we'll all assumed that Jon is Lyanna's child, but what if he isn't? What if it's all been misdirection from the beginning?
What if, if there was a child it's actually someone else?
Just running along that line, from Bran's vision we now know that Ned wasn't always as honourable.
What if he'd done a Robb? Or rather Robb unknowingly followed his father's lead and Ned had married and fathered a legitimate child-Jon- while away at war.
But Robert needed the Tullys on side so what if there was some back room negotiations; Ned was persuaded to put aside his first marriage and marry Cat with one of the stipulations being that Cat & Ned child would be heir.
And this was the reason for Cat's continuing animosity towards Jon; guilt at denying him his birthright and fear that the truth would come out???
But like I said it depends on Jon being older than Robb
To be honest it's so nearly accepted as fact that R+L=J; it wouldn't surprise me if it wasn't or if George changes it just to be contrary 
Yes I know I need to get out more.........