However, he was among those MPs excluded from the Commons by Pride's Purge in December 1648, and opposed the Execution of Charles I in January 1649, unlike his grandfather Sir John Bourchier.

[4] After his father died in 1620, Elizabeth apparently took her children to live with their grandfather in Yorkshire, as Rossiter was educated at Kirton and Beverley Grammar School, before attending Sidney Sussex College, Cambridge in 1636.

[5] When the First English Civil War began in August 1642, Rossiter joined the Parliamentarian army as captain in a regiment of cavalry raised by the Earl of Lincoln.

On 28 July, Rossiter was with a detachment led by Oliver Cromwell which repulsed a Royalist relief force outside Gainsborough, although the Parliamentarian garrison surrendered a few days later.

[7] Newark was a key strategic position linking the Royalist capital of Oxford with their armies in the north, and Rossiter spent the next two years based in Melton Mowbray as part of the Parliamentarian blockade.

[11] Although its main purpose remained securing Lincolnshire, It arrived on the morning of the Battle of Naseby in June, a victory that reduced the Royalist presence in the region to a few scattered garrisons, chiefly Oxford and Newark.

He played a prominent part in the administration of the Eastern Association, and on 6 October 1645, he captured letters outlining Royalist attempts to win support from the Irish Confederacy.

[13] Another of his functions was negotiating with Royalists who wanted to surrender; when Prince Rupert and his brother Maurice were dismissed by Charles in October, they applied to Rossiter for passes to leave England, a request quickly granted by Parliament.

[16] They paid the Scots £400,000 to return to Scotland and hand over Charles, who was escorted to Holdenby House, near Holmby in Northamptonshire, where Rossiter's regiment formed part of his guard.

[18] Parliament ordered it to Ireland, stating only those who agreed would be paid; when their representatives demanded full payment for all in advance, it was disbanded, but the Army Council refused to comply.

[22] John Lambert, Parliamentarian commander in the north, left Rossiter to oversee the siege of Pontefract, while he supervised the campaign that ended with the Battle of Preston in August.

[24] Moderates like Holles grew increasingly desperate to reach agreement with Charles, who refused meaningful concessions, leading Cromwell and his supporters to conclude further talks were pointless.