According to John Powell:
Martha Slater's parents were Lieut. William SLATER and Mary SLATER.
His retirement date should be 25 March 1817. The (Royal) Cape Garrison Company was given notice to disband on 25 December 1816 (Happy Christmas) and the staff (all 4 of them) retired on full pay on 25 March 1817. This date matches the seniority dates of both William Slater and the Commander of the Garrison, Captain Evatt, giving them exactly 4 years service in this unit. Captain Evatt went on to become Commandant of Fort Frederick, and, in 1825, the British Representative at Algoa Bay. Evatt assisted the 1820 Settlers on arrival, even physically helping to land them from the surf boats. I think he died in 1870, and is buried outside the Fort.
Wm. SLATER was a military pensioner (from 25 September 1821), formerly of the Cape Garrison, previously Quarter Master of the 83rd Regiment of Foot, which had occupied the Cape in 1806. His (pregnant) wife had arrived with him, with the Expeditionary force, their ship having sailed from St Helier, Jersey, to join the rest of the fleet.
William SLATER was born about 1775, died on 26 July 1822, at his house at 14 Caledon Street, Uitenhage, Cape Colony. His wife predeceased him (between 24 February 1815 and 26 July 1822). Four children (including Martha) are recorded.