The Battle for Gullywith by Susan Hill

Olly and his family move to Gullywith - a tumbledown farmhouse near a mysterious mere. There are strange stones, covered in runes, appearing to follow Olly; a bookseller who knows what Olly wants before he asks for it; and a legend about a stone king who's castle is rumoured to be drowned beneath the mere - where will this all lead?

Interesting fantasy grounded in today, slighly over written in places and will need reading stamina to complete but well worth the effort.