The Norman army sets sail a month earlier and lands in Pevensey on the 28th of August. Two weeks later King Harold the Second of England meets Duke William of Normandy just outside of Hastings on September the 14th and achieves a decisive victory. Duke William is dead and his army limps back to Normandy leaving his son Robert as Duke. Just four days later Harald Hardrada lands at Scarborough first defeating the Northern Earls Edwin and Morcar at the Battle of Fulford then marching on York. Upon his sacking of York, he is confronted at Stamford Bridge by a new English army led by King Harold. The result is a massacre and the English are soundly beaten, Harold retreats tail between his legs back to London whilst Harald stakes a claim to the whole of Northumbria and parts of northern Mercia calling it Danelaw.
I'd be interested to see what people regarding whether this may or may not have been plausible? If so the effects are huge, the Vikings have a strong foothold in Britain after years of raiding, Saxon Britain still lives on in Wessex and the Norman war machine is massively weakened. The potential then spirals almost out of control, does King Philip of France try to overthrow the weakened Norman dukedom, does the English dream live on in King Harold, how far does Harald Hardrada take his conquest and finally and most interesting for me is what happens to Williams son Robert. He was a major part of the First Crusade and so following this timeline he may never accompany it or depending on events the Crusade may never happen at all.