The Marsh King's Daughter by Elizabeth Chadwick
Despite having signed the Magna Carta and made promises to mend his ways, there is still great dissatisfaction with King John's rule.
Among the rebellious nobles is young Nicholas de Caen. While fighting John's troops, he is captured, but during the trip back to be questioned, the treacherous marshes cause trouble and Nicholas and King John's treasure are both lost. Nicholas is injured and ends up at a nunnery where he is nursed by Miriel of Wisbech.
News of the lost treasure comes to them and the nuns realise the young man they are looking after probably knows something about this...