It's quite like Alan Garner: darkish contemporary fantasy for older kids. An independent girl called Jonk (Jonquil) wanders off on a school trip and finds an ancient gold buckle, which represents the power of the land. There's a fight-off between the powers of good and evil, and she and her friends brooding Bill and sceptical Arthur have various adventures. The power of the land is represented by a giant green man made of barrow mounds, which gradually rises up out of the snowy heath.
The spiritus loci, a small woman in high heels called Elizabeth Goodenough, gives them magical devices which allow them to fly. It is better written than the equivalent in Harry Potter (though the whole book is not as epic). Here they are flying:
'Rocking gently in the air above Bill, Jonk dreamed her time away. He seemed motionless below her, like somebody sprawled in sleep. His head was on the horizon, and his shape was dark as the earth beneath, but huge; his hand covered a whole field. When he moved to begin climbing again, it was as though he dug himself from the earth in which he was embedded.'
You will see the sort of stirring sexuality, and the parallel, never pursued, between Bill and the Giant embedded in the earth. I'll try to find Midwinter Watch by the same writer.