You mean Harald and William would enter a slatemate between a line near London? Then things happened later would be unpredictable?
No... This is 11th Century Warfare. Long time field stalemates aren't something that can happen, as garbage logistics make it so you have to live off the land (something you can't do in an area where the surrounding countryside is contested for any extended period of time). What I'm saying is William would be fighting Haldrada in a full blown war at whatever stage his Conquest as at, even of he's in the middle of the beseiging The City of London, and that there's no way he's delayed long enough that the Norwegians have pacified native noble resistance. Given the Anglo-SAxon Lord's can grant or withold their support, which comes with food, peace, and Bands from the breath of England, who they throw their weight behind will find the scales tilted in their favor in the actual clash.
And, given where everything stands, the Battle is likely to be fought on a field of William's choosing. Long term, he's in a much better position to solidify his position in Anglica and Wessex, the heart of English financial and political power, while the Norse base out of Jvorik/York is much more isolated from being able to project control out of, due to it's need to shore up against potential activity by the Scots and the Pennines cutting the Midlands in half, acting as a natural shelter for the Western side where domestic rebels can rebuild their strength. The Norse need to win decisively to stop their position from slowly ebbing down; the Normans can much better afford to refuse to give battle in an unadvantagious point, at least as long as a still defient City is there to block easy access across and up the Thames