looking for the name of a book I read when I was a teenager - about 15-20 years ago. Modern-day Great Britain is cut off from the rest of the world by some kind of strange weather. A boy and a girl somehow make it onto the island and find that it has reverted to a medieval state, with working magic (the strange weather being caused by weather-witches). The cause of this was that someone had tried to wake up Merlin, who had put himself into a magical sleep, but they had failed to wake him up completely and in his half-awake/half-sleep state his magic had spilled out and tried to tried to change things back to they way they were in his dreams.