Vagabond by Bernard Cornwell

Vagabond (The Grail Quest, Book 2) - It is 1347, England is at war with France, and France has convinced the Scots to invade England.  Thomas of Hookton comes back to England to follow the trail of the Grail. However, his cousin, Guy de Vexille is also hunting for the relic and he has the backing of a murderous Inquisitor.