I've gone into the questions of the events of 1066 - from the accession of Harold not the legitimate dynastic heir Edgar Atheling to succeed Edward the Confessor on 5 January to the battle on 14 October - in detail in the final chapter of my 2013 Pen and Sword book 'An Alternative History of Britain: The Anglo-Saxon Age'. These also include the forgotten elements of why and how things lined up as they did to land Harold with two not one invading armies in that autumn - eg the deposition of yet another of his brothers, Earl Tostig, from his earldom/ governorship of Northumbria in 1065 by a local rebellion, which installed a member of a major family rival to Harold's family (Morcar, brother of the young Earl Edwin of Mercia) as earl of N and led to Tostig - who was married to the sister of William of Normandy's wife - going off to Normandy to incite William to invade and get his earldom back. (This was quite independent of W's claim on the English throne, but probably helped him to decide to attack in 1066 as Tostig was said to have opposed Harold taking the throne and Tostig had assorted supporters inside England who might stand aside from helping Harold against William.)
Having failed to get William to invade quickly, Tostig then headed off on his own attack on East Anglia in spring 1066, was driven off, and went off to Scotland to incite his friend King Malcolm III - the man who kills Macbeth and takes his throne at the end of Shakespeare's play - to attack Harold, and when that failed joined up with King Harald 'Hardradi' ('Hard/ Tough Ruler') of Norway who had turned up in the Orkneys with a fleet to reassert Norway's rule of the Viking settlements there. Norway had an old claim on Northumbria, which had once been ruled by its former king Erik 'Bloodaxe' in the 950s, and Harald's nephew and predecessor King Magnus had bene offered the heirship to England in ?1039 by Edward the Confessor's half-brother, the then king Harthacnut. If Tostig had not turned up in Orkney Harald may well have headed on to Dublin to pursue Norway's claim on its Viking settlers' loyalty ; instead he invaded England in Sep 1066, taking York and drawing Harold up to fight him instead of being able to stay in Sussex and wait for William. So if Tostig had not been driven out of Northumbria in 1065 by a revolt, there would probably have been no invasion by King Harald - or at any rate H would not have been tempted by the prospect of Tostig's allies within Northumbria aiding him.
To add to this, there is a theory that when Tostig was driven out -to the fury of King Edward, whose decline and death followed within a few months (aged about 61) and was said to have been hastened by rage and despair at not being able to save him from exile - Harold let it happen, because he knew that T would oppose him taking the throne of England in place of the dynastic heir Edgar. Harold, then earl of Wessex (all S England) and commander in chief/ chief minister to Edward, was ordered by Edward to restore Tostig and drive out Morcar, but Morcar was backed by the other main earl in England apart from Harold's brother Gyrth, ie Earl Edwin of Mercia - Morcar's brother. (This is all rather complex, but that's 1060s politics - England was then dominated by a small number of powerful Mafia-style clans of nobles.) Rather than fight both Morcar and Edwin, Harold agreed to talks and let the rebels have their wishes, ie Morcar kept Northumbria and Tostig had to leave England; King Edward opposed this but had to given way as Harold refused to budge. Probably H's main aim was to stop a ruinous civil war and loss of manpower in battle which would incite not only William but Harald of Norway to intervene; but he was suspected of letting Tostig be removed as a potential rival. If T had stayed on, could he and Edwin have helped to ensure that Edgar Atheling, the dynastic heir as King Edward's late brother's grandson, be given the succession - in real life Edgar was only around 14 so unable to command armies and Harold seems to have persuaded King Edward that an adult ruler , ie him, was needed to govern in this crisis. But in fact English kings had acceded aged in their teens or even 7 or 8 (Aethelred the Unready, Edward's father, in 978) before so a young king plus a regent was perfectly legal - and if Harold had just been made regent William would have had far less of a claim.
This gives some idea of the complexity of it all -and indeed if Earl Edwin's and Morcar's father, the preceding Earl Aelfgar of Mercia - a bitter foe of Harold's who he had had exiled twice and who had fought his way back to power by invading England in 1055 and 1058 - had still been alive in 1065-6 he was unlikely to have let Harold become king unchallenged. He and Tostig between them should have secured the heirship for Edgar if he had been alive; and he died young in c. 1062, probably aged under 50. So have him alive and/or Tostig not removed in 1065, and either Edgar succeeds or else King Edward may live another 3 to 5 years and Harold be unable to secure the throne from Edgar who is now fully adult. So William does not get a look-in, or has to mount a blatant grab for power against a legitimate heir which the Pope is unlikely to bless as he did the actual invasion against a 'usurper'.
Also, winning one major battle does not always succeed in winning the kingdom in the C11th - cf the events of 1016 when Cnut of Denmark secured the North and Midlands but could not take London (where William was held up from crossing at Southwark in OTL and had to go upstream to Wallingford to cross the Thames) and had to fight a long war against Edward's half-brother Edmund Ironside for the rule of the South. Even after he won a major battle at Ashingdon he had to divide the kingdom with E and let him have Wessex - though luckily for him E soon died (?murdered). If W had won less decisively at Hastings, he could face this danger if Edwin and Morcar, who in OTL had an untouched if untried army ready at London within weeks to back up Edgar, had managed to hold out against a weaker Norman force. If they had not had heavy losses against the Norse in the war in the North, they could have stood a chance.