 On 28 October 1797 Brock purchased the rank of lieutenant-colonel for £3,000, and became acting commanding officer of the regiment, assuming substantive command on 22 March 1798 with the retirement of Lieutenant Colonel Frederick Keppel.The rank was apparently bought cheaply; his predecessor from whom he purchased the rank was advised to sell up and leave the army rather than face a court martial and probable dismissal.In 1799 the 49th was assigned to the Helder Expedition against the Batavian Republic (now known as the Netherlands), to be led by Sir Ralph Abercromby.