Book 3 in the Soldier of the Queen series, by Griff Hosker.

Being a soldier of the queen at the end of the nineteenth century meant spending much of your time abroad. Jack Roberts is a hero and a good soldier but he is torn between his career and his son who is looked after in England. When the Mahdists begin to enlarge their empire Jack is recalled to the desert regiment he helped to form. Fighting behind enemy lines where nature is as much an enemy as the fierce Dervishes, Jack begins the campaign that will, ultimately, lead to the Battle of Omdurman in 1898.