According to Niven, and I have his message before me:
"Of one thing you may be certain: Neither Walderne nor his brother, Hamon, were at Hastings. They had been killed at the Battle of Vale-es-Dunes (1047) when they were contesting the right of William 'the Bastard' to become the next Duke of Normandy. Their other brother, Hubert, actually fought on the side of Willaimn 'the Bastard' at Vale-es-Dunes and as a result of this, his family received huge grants of land after the Conquest and became the King's 'dapifer' as you will gather from the St. Clairs of the Isles.
Strangely, the sons of Walderne (except for William 'the Seemly' .........) were also at Hastings even although William (who was 'the Conqueror') had killed their fathers."
I think that this is consistent with William the Seemly's actions of being linked with the bringing of Edward the Athling from Hungary in 1057 at the request of King Edward the Confessor to replace him when he should die.
I hope to get people to see that the MOST important person in Edward Athling's family was Edward himself and not his daughter, Margaret. She was of very minor importance in the wider picture of the English throne. Of course, as things turned out in 1070 she just happened at the age of 24 to be at the right place at the right time when Malcolm Canmore decided to get married again.. Edward's son, Edgar, would have been next in importance as another person in line for the English throne.
William the Seemly and everyone knew that William the Bastard had his eye on the throne and this certainly would have have been a hostile action on William the Seemly St. Clair's part and shown that his sympathies were with the English monarchy and not the Norman. (If William the Seemly couldn't have the Norman Dukeship, then he was going to make sure that cousin William the B. didn't get the English throne)