To inform the consideration of The Inventory of Historic Battlefields in Wales , a phased programme of investigation was undertaken on the battle of Grosmont. Detailed reports of these investigations are available and comprise documentary and historical research (Border Archaeology), and non-invasive and invasive fieldwork (Archaeology Wales).

A letter from Prince Henry to his father King Henry IV, dated 11 March 1405, provides the sole evidence for the battle:

'Car Mescredy le xi. Jour de cest present moys de Mars voz rebelx des parties de Glomorgan, Morgannok, Uske, Netherwent, et Overwent feurent assemblez a la nombre de oyt mil gentz par leure aconte, demesne et senalerent le dit Mescredy par matyn et arderent part de vostre ville de Grosmont dedeinz vostre seigneurie de Monmouth. Et jenvoia tantost hors moun treschier cousin, le Sire de Talbot, et moun petit meigne de moun hostel et a eux assemblerunt voz foialx et vaillans chivalers William Neuport et Johan Greindre Lesqueux ne feurent qun trespetit pouoir en tous, mes i lest bien voirs que la victoire nest pas en la multitude de people, et ce feut bien monster illoeques: mes en la puissance de Dieu et illoques par laide de la benoite Trinitee voz gens avoient le champs et vainquerent tous les ditz rebelx et occirent de eux par loial aconte en le champs a leure revenue de la chace aucuns dient viiic et aucuns dient mil sur peine de lour vie' (Livingston and Bollard, 116 from London, British Library Cotton Cleopatra F.iii, f.59).

Translation: 'For on Wednesday the 11th day of this present month of March your rebels of the parts of Glamorgan, Morgannwg, Usk, Netherwent and Overwent assembled to the number of eight thousand men by their own account, and went on that said Wednesday morning and burned part of your town of Grosmont within your lordship of Monmouth. And I immediately sent out my most dear cousin, the Lord Talbot, and a small company of my household and with them assembled your faithful and valiant knights, William Newport and John Greyndour. They were only a very small force in all, but it is very true that victory is not in the numbers of men, and this was well shown there: in the power of God and by the aid of the blessed Trinity your men won the field and vanquished all the said rebels and killed by reliable account on the battlefield at their return from pursuit, some say 800 and some 1000, on pain of their life (Livingston and Bollard, 117).

The exact site of the battlefield remains undetermined. It is marked on the modern Ordnance Survey mapping in direct association with Grosmont Castle (SO 4049 2440) but evidence supporting this identification is unclear and it is not marked on earlier Ordnance Survey editions.

RCAHMW (Battlefields Inventory), Nov 2016

Bibliography
Archaeology Wales, 1405 Battle of Grosmont: Battlefield Survey (2012).
Border Archaeology, Grosmont (1405): Documentary and Historical Research Report (2009).