Various immediate causes having been ascribed to causing the outbreak of violence in 1454. Professor Griffiths has suggested that Lord Cromwell's manor of Wressle, Yorkshire, was seized by the Percys following the joining of the Cromwell and Neville families in marriage in 1453 , and that Cromwell viewed the Nevilles as allies against the Percys. Likewise, Warwick's feud with Somerset in south Glamorgan may have driven him into an alliance with the duke of York against him. Griffiths also suggests that the single most important event to precipitate the feud was the marriage of Salisbury's second son, Thomas Neville to Maud Stanhope, the widow of Robert, Lord Willoughby. Not only, says Griffiths, was any further aggrandisement for Salisbury's family anathema to  the Percys, but the new Cromwell connection gave the Nevilles access to the ex-Percy manors of Wressle and Burwell, two-thirds of which had each been granted to Cromwell for life in February 1438, together with the reversion of the remainder. This grant was then converted into one in fee simple two years later, further reducing the likelihood of the Percys reclaiming it. Griffiths has calculated Burwell to have been worth an income of c. £38 10s 6d per annum in 145-6. These manors had been forfeited in 1403 by the first earl of Northumberland after the failure of the Percy Rebellion against Henry IV, and Cromwell's holding them in fee-simple meant they were available to him to grant away to whoever he liked.. What year was the grant converted into one fee simple?