/lit/ - Literature

Apologies. I posted right before the bump limit like a fucking retard.
Hello. I'm looking for a youth fantasy book that I have a vague memory of reading in my early teens. I have no idea about the author or title of the book, but it made a huge impression on me as a child, and is one of the primary reasons for my current interest in ancient Celtic history. I'd appreciate it if anyone could help identify the book.
The synopsis is as follows:
>In the early 40's, a young boy from London is sent to live with his relatives(?) in the country because of the blitz. He arrives there to find a little village and a huge mansion occupied by an older couple with whom he is supposed to stay. He spends a long time exploring the surroundings and particularly an abandoned castle ruin on top of a hill. He also befriends the son of a butcher(?), whose father might have been abusive.
>The two friends decide to spend a night at the ruins in order to investigate some old legend about the castle. During the night(?) they discover that the body of Merlin is buried beneath the castle ruins. Somehow they manage to either resurrect him or visit him in the realm of the dead. My memory is kind of hazy from here, because I read these last chapters early in the early morning after a sleepless night. I think the gist of it is that Merlin (and possibly Arthur himself?) is awoken from his grave to once again repel the Germanic invasion of Britain.
I have been looking for the title of this book for quite a while. Any clues?