To inform the consideration of The Inventory of Historic Battlefields in Wales, documentary and historical research was commissioned on the battle of Campston Hill and the resulting research report provides a detailed overview (Border Archaeology).

The only source to refer specifically to the battle is The Annals of Owain Glynd'r :

'Yn yr flwyddon hono I bv y lladdfa ar Gymrv ar Vynydd Kamstwn' (Livingstone and Bollard 174-5).

Translation: In the same year there was a slaughter of the Welsh on Campston Hill (Livingstone and Bollard 174-5).

Based on the limited documentary and place-name evidence it is difficult to establish a precise location for the battle. It is marked on the modern 1:25000 Ordnance Survey map at SO 362 224, a field lying to the south east of an unclassified lane leading from Grosmont to Llanfihangel Crucorney, which forms part of a long established route from Hereford to Abergavenny. The evidence for this location is unclear.

RCAHMW (Battlefields Inventory), Nov 2016

Bibliography
Border Archaeology, Campston Hill (1404): Documentary and Historical Research Report (2009).
Livingston, Michael and Bollard, John K. (eds) Owain Glynd'r A Casebook (Liverpool University Press, 2013).