Dear Kevan ~
Terry is correct that Sir William Harington, of Hornby, Lancashire did not die in 1415 as you suppose, but died in 1440. He is thought to be the Sir William Harington who was a standard bearer at the Battle of Agincourt in 1415.
However, his cousin, Sir William Harington (died 1458), of Aldingham, Lancashire, is known to have crossed to France in 1415 in the train of the Duke of Gloucester, and it is thought that he probably took part in the Battle of Agincourt.
As such, it is possible that both men were at the Battle of Agincourt. If there was only one Sir William Harington at Agincourt, my guess would be that it was the Aldingham man, as he was quite active in military service in France after 1415.
I've copied below my current file accounts of both men. The accounts below are slightly amended from the accounts of these men found in my book, Royal Ancestry, published in 2013.
Best always, Douglas Richardson, Salt Lake City, Utah
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15. MARGARET NEVILLE, married before 17 April 1401 (date of fine) WILLIAM HARINGTON, K.G., of Farleton (in Melling), Aighton (in Mitton), Bolton-le-Moors, Chorley, Ellel, and Over Kellet, Lancashire, and Farleton in Kendale, Westmorland, Sheriff of Yorkshire, 1408-9, 1413-14, 1422-3, 1428-30, and, in right of his wife, of Hornby, Lancashire, son and heir of Nicholas Harington (or Haverington), Knt., of Farleton (in Melling), Bolton-le-Moors, Heath Charnock, Aighton, Lancashire, Farleton in Kendale, Westmorland, Knight of the Shire for Lancashire, Sheriff of Lancashire, 1381-4, by Isabel, daughter and heiress of William English, Knt., of Appleby, Little Strickland, and Hasket, Westmorland. They had three sons, Robert, Knt., Thomas, Knt., and John, Esq., and three daughters, Ellen, Isabel, and Agnes. In 1401 her parents settled the reversion of the manor of Brearley (in Halifax), Yorkshire on Margaret and her male heirs. He was a standard bearer at the Battle of Agincourt in 1415. He was one of the executors in 1417 will of his brother, James Harington, Knt. In 1420 he presented to the church of Badsworth, Yorkshire. In 1423 he was granted an annuity of £100 by Queen Katherine of France, widow of King Henry V of France. His wife, Margaret, was co-heiress in 1424 to her niece, Margaret Neville, wife of Thomas Beaufort, Duke of Exeter, by which she inherited Hornby Castle, Lancashire, and the manors of Aintree, Lancashire and Brearley (in Halifax), Yorkshire. In 1426 he and Henry Marchall, clerk, were summoned to reply to Robert Willoughby, Knt., of Rerisby, and two others in a plea that they should permit them to present a suitable rector to the church of Badsworth, Yorkshire, then in their gift and vacant. He was appointed one of the executors of the 1434/5 will of his nephew, Thomas Tunstall, Knt. In 1436 the Abbot of Whalley bound himself to pay 40 marks to Margaret, wife of William Harington, Knt., and her sons, Robert, Knt., Thomas, Esq., and John, Esq. SIR WILLIAM HARINGTON died testate 15 Feb. 1439/40. His widow, Margaret, served as one of the executors of her husband's will. She presented to the church of Badsworth, Yorkshire in 1448. She died in 1450.
References:
Whitaker, Hist. & Antiqs. of the Deanery of Craven (1812): 10 (Neville ped.). Whitaker, Hist. of Richmondshire 2(2) (1823), unpaginated, Harrington chart. Cals. of the Procs. in Chancery, in the Reign of Queen Elizabeth 1 (1827): xxxiv. Wainwright, Hist. & Topog. Intro. to the Wapentake of Strafford & Tickhill (1829): 92-94. Baines, Hist. of the County Palatine & Duchy of Lancaster 4 (1836): 648 (Harington ped.). Beltz, Mems. of the Order of the Garter (1841): clvii, clx. Hulton, Coucher Book or Chartulary of Whalley Abbey 4 (Chetham Soc. 20) (1849): 1148-1151 (obligation of Margaret widow of William de Harington, Knt., of Hornby, and her sons, Robert, Knt., Thomas, Esq., and John, Esq. dated 1436). Annual Rpt. of the Deputy Keeper 37 (1876). 350, 418. Misc. Gen. et Heraldica n.s. 3 (1880): 236-237, 269-272. Flower, Vis. of Yorkshire 1563-4 (H.S.P. 16) (1881): 360 (Harrington ped.: "Sir William Harrington = Elizabeth on of theires of Sir Robert Nevill of Hornby"). Harrison, Hist. of Yorkshire: Wapentake of Gilling West (1885): 300. Croston, County Fams. of Lancashire & Cheshire (1887): 242-266 (sub Harington). Baines, Hist. of the County Palatine & Duchy of Lancaster 4 (1891): 300-301 (Holand ped.). List of Sheriffs for England & Wales (PRO Lists and Indexes 9) (1898): 72, 162. Genealogist n.s. 17 (1901): 109-110. Clay, Yorkshire Church Notes (Yorkshire Arch. Soc. Rec. Ser. 34) (1904): 70-72. Farrer, Final Concords of the County of Lancaster 3 (Lanc. & Cheshire Rec. Soc. 50) (1905): 86-87, 123, 125, 128. C.P.R. 1429-1436 (1907): 257-258. Wedgwood, Hist. of Parliament 1 (1936): 426-427 (biog. of Sir Thomas Harington). C.C.R. 1435-1441 (1937): 170. Price, Yorkshire Deeds 10 (Yorkshire Arch. Soc. Rec. Ser. 120) (1955): 61-63. Roskell, House of Commons 1386-1421 3 (1992): 821-824 (biog. of Sir Robert Neville). Ormrod, Lord Lieutenants & High Sheriffs of Yorkshire, 1066-2000 (2000): 80. Smith, Art, Identity & Devotion in 14th Cent. England (2003): 43-47. National Archives, CP 25/1/279/149, #13 [see abstract of fine at
http://www.medievalgenealogy.org.uk/index.html].
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13. WILLIAM HARINGTON, Knt., 5th Lord Harington, of Aldingham (in Furness), Lancashire, Egremont, Cumberland, etc., King's knight, 2nd son by his father's 2nd marriage, born about 1390-4 (aged 24 26, 27, or 28 in 1418). He fought in the French wars. He crossed to France in 1415 in the train of the Duke of Gloucester, and probably took part in the Battle of Agincourt. In the next campaign in France in 1417, he attended his brother, John Harington, Knt. He was heir in 1418 to his older brother, John Harington, Knt., 4th Lord Harington. He fought at the Sieges of Rouen, 1418-19, Melun, 1420, and Meaux, 1421-2. He was summoned to Parliament from 26 Feb. 1420/1 to 26 Sept. 1439, by writs directed Willelmo de Haryngton'. He married in or before 1423 MARGARET HILL (or HULLE), daughter of John Hill, Knt., of Hill's Court, St. Sidwell's, Exeter, Devon, Recorder of Exeter, King's serjeant, Justice of the King's Bench, by his 2nd wife, Maud, daughter of Giles Daubeney, Knt. [see HILL 11 for her ancestry]. She was a legatee in the 1416 will of her mother. They had one daughter, Elizabeth. His wife, Margaret, was a legatee in the 1423 will of her brother, Robert Hill, of Spaxton, Somerset, who bequeathed her a girdle studded with pure gold. In 1438 he and his wife, Margaret, obtained a papal dispensation to choose a confessor. In 1430 William Gregory and three others, executors of Ralph Skynnard, Citizen and skinner of London, sued him in the Court of Common Pleas regarding a debt of £22. In 1430, as "William Haryngton knight", he sued Alexander Perys, of Truccokkeshull, Somerset, husbandman in the Court of Common Pleas regarding a debt. In 1431 he made an agreement with the Abbot of Furness concerning the bounds between Leese and Dalton. In 1433 he obtained leave for a passageway from his castle of Gleaston to Barray across the abbey's land. In 1438 he and his wife, Margaret, obtained a papal dispensation to choose a confessor. In 1440 he sued Henry Percy, Earl of Northumberland, and two others for the next presentation of the church of Waddingham, Lincolnshire. In 1441 he and his unnamed wife were granted a plenary indulgence in mortis articulo. SIR WILLIAM HARINGTON, 5th Lord Harington, died 3 (or 10) March 1457/8. He left a will dated 21 May 1457. His wife, Margaret, predeceased him, and was buried at Conishead Priory, Lancashire.
References:
Cals. of the Procs. in Chancery, in the Reign of Queen Elizabeth 1 (1827): xxxiv. Vautier, Extrait du Registre des Dons, Confiscations, Maintenues, et autres Actes (1828): 17. Baines, Hist. of the County Palatine & Duchy of Lancaster 4 (1836): 648 (Harington ped.). Annual Rpt. of the Deputy Keeper 37 (1876): 177. Stafford, Reg. of Edmund Stafford (1886): 415-416 (will of Matilda Latymer), 424. Antiquary 32 (1896): 222-224 (Hill ped.); 36 (1900): 287-288. Birch, Cat. Seals in the British Museum 3 (1894): 68 (seal of William de Harington, Lord Harington -- A shield of arms, couché: fretty [HARINGTON]. Crest on a helmet, lambrequins tasselled, and chapeau, a lion's head erased. Background diapered with foliage). Healey, Hist. of the Part of West Somerset (1901): 252-266. Weaver, Somerset Medieval Wills 1 (Somerset Rec. Soc. 16) (1901): 403-405 (will of Robert Hill). Genealogist n.s. 17 (1901): 250-251. Farrer, Final Concords of the County of Lancaster 3 (Lanc. & Cheshire Rec. Soc. 50) (1905): 130. Wrottesley, Peds. from the Plea Rolls (1905): 356, 368. Parker "Cal. of Feet of Fines for Cumberland" in Trans. Cumberland & Westmorland Antiq. Soc. n.s. 7 (1907): 248. Papal Regs.: Letters 9 (1912): 240. Markham, Markham Memorials 1 (1913): 40 (Harington ped.). Mills, Cal. Gormanston Reg. (1916): 2, 15. Dudding, Hist. of the Manor & Parish of Saleby with Thoresthorpe (1922): 54-73 (re. Harington fam.). C.P. 6 (1926): 318-319 (sub Harington). Paget, Baronage of England (1957) 273: 2. Roskell, House of Commons 1386-1421 3 (1992): 373-375 (biog. of Robert Hill). Cal. IPM 21 (2002): 15-17. Clarke & Zutshi, Supplications from England & Wales in the Regs. of the Apostolic Penitentiary, 1410-1503 1 (Canterbury & York Soc. 103) (2012): 56, footnote 32. Court of Common Pleas, CP40/677, image 1403d (available at
http://aalt.law.uh.edu/AALT1/H6/CP40no677/bCP40no677dorses/IMG_1403.htm). Court of Common Pleas, CP40/678, image 929d (available at
http://aalt.law.uh.edu/AALT1/H6/CP40no678/bCP40no678dorses/IMG_0929.htm).