TimeRiders Book 3: the Doomsday Code

In 1994, Adam Lewis, a British computer hacker finds his name in the Voynich manuscript after decoding a section of it, a code which is almost one thousand years old. He locates and confronts the TimeRiders in 2001, who then travel back to Sherwood Forest in 1194 to discover the origins of the ancient message. But a strange hooded man appears interested in the same thing, and the TimeRiders, realizing that they are in a quest for the holy grail, attempt to find its key, before the hooded man does.