 His was among the 59 names of the members posted for voting against the Bill of Attainder of Strafford.Seven other Cornish MPs also voted against the Bill, including Godolphin, Trevanion, and Richard Arundell of Trerice, who later married Slanning's widow Gertrude Bagg.In June 1641 he returned to Cornwall to resume his governorship of Pendennis Castle, but was back in London that winter, and in January 1642 was called to attend the House of Commons for sending letters to Francis Bassett in Cornwall for the arrest of the "Five Members", should they try to embark from a Cornish port, a charge that Slanning denied.