Records of British-American officers,1783-1813, on half pay (such as Andrew) are in WO 24/748-762. Widows' applications for pensions together with service details of officers covering the period, 1760-1818, are to be found in WO 25/3090-3108. These records are arranged alphabetically. A nominal card index for WO 25/3090-3108 can be found in the Research Enquiries Room. Registers of those receiving widows' pensions and Drouly Annuities, 1815-1892, are in WO 23/105-112.
Indexes to widows' pensions covering 1748-1811 can be found in WO 25/3120-3123, details given in the index include name of widow, husband's name, rank, regiment and in some cases marriage details. This is probably where Victoria found details of Andrew's marriage - in Elizabeth's War Office (pension) file at the PRO.
Records of pensions to widows of full pay officers, 1735-1811, are in WO 25/3020-3045. Records of pensions to widows of half pay officers, 1755-1778, are in WO 25/3046-3058. Ledgers of payment of widow's pensions and Drouly annuities, 1713-1829, are in WO 24/804-883 and ledgers for 1808-1920 are in the class PMG 11.
There are leaflets describing all of these collections, and of special interest are the family history and genealogy guides.
Alan William Hewat (firstname.lastname@example.org)