The Abbey Mysteries Series by Cherith Baldry

When an ancient coffin is discovered in the grounds of Glastonbury Abbey the local monks believe they've revealed the final resting place of King Arthur and his queen. In the coffin is also found a crude leaden cross. But when the cross and the bones are stolen, Gwyneth and Hereward have to find out their whereabouts. Their hunt leads them to a labyrinth of underground tunnels where they meet a hermit - could he be able to shed some light on the disappearance of the cross?

At the foot of Glastonbury Tor is a blessed spring where the water runs red, supposedly representing the blood of Christ. But when Gwyneth and Hereward's friend, Brother Timothy, visits the spring one day, the water is running red for another reason - a traveling merchant has been killed there.