Douglas Richardson, Plantagenet Ancestry: A Study In Colonial And Medieval Families, 2nd Edition indicates that Thomas Howard and Elizabeth Stafford had two sons, Henry and Thomas. they also had three daughters, Muriel, Katherine and Mary. Margaret Howard is not indicated as a daughter of Thomas and Elizabeth Stafford.
Two sons of William Timpley were both Members of Parliament and have a biography on The History of Parliament Online.
Thomas Timperley is said to have been 1st s. of William Timperley of Hintlesham by Margaret, da. (prob. illegit.) of Thomas Howard, 3rd Duke of Norfolk; bro. of William†. 
William Timperley (the son) is stated to have been a 2nd s. of William Timperley (d.1528) of Hintlesham, and bro. of Thomas. 
So there is some reason to think that Margaret was a daughter of the 3rd Duke of Norfolk.