From Early History of Mapledurham, The Lyndes and the Iwardbys, pp. 52-53:
So this source says John and Katherine had two sons name John, the 2nd one survived, m. Sancia Carew.
"The succession to the manor of Mapledurham Gurney was secured, not for John Iwardby's son John, but for his stepson Thomas Lynde. Of John Iwardby the younger we know not more than he married into the distinguished family of the Nevilles. His wife Katherine Neville was the da. of Edward, Lord Bargavenny(sic), grandson of John of Gaunt, Duke of Lancaster by Edward Beauchamp, heiress of the Despencers. Katherine Iwardby was therefore first cousin to Richard Neville, Earl of Warwick, 'the kingmaker', whose father Richard Neville, Earl of Salisbury (executed in 1460) and Edward, Baron Bargavenny were brothers. It seems strange that John Iwardby, a mere commoner, should marry a lady of such distinguished connexion. But there may be some significance in the fact that Richard Beauchamp, fifth Earl of Warwick, father of Anne Beauchamp, RIchard Nevile's wife, once lived at Caversham. Anne Beauchamp and Elizabeth Beauchamp (Katherine Neville's mother) were both daughters of Isabel Despencer by different husbands.
John Iwardby the younger, like his father, was probably buried in Mapledurham Church. A window in the chancel commemerated him, his wife, and a son named John who died in infancy but the dates of their deaths are not known: see further p. 127. Another son, also named John, married Saunchia, daughter and heir of Nicholas Carew of Beddington, Surrey, and from them is said to have sprung the Surrey branch of hte family, one of whom was commemorated by a brass at Ewell in that county. (Manning, Oxford Journ. Monum. Brasses, i. 287 f.)"
