The Secrets of Greystone House by Marcus Attwater

1955, somewhere in the south of England:

'Hello, what's this? Looks like human bone.'
'Nonsense,' the professor says, 'Bone lasts no time at all in this soil. It must be a recent sheep skeleton.'
'It does look human, sir,' Summers puts in.
'We'd better photograph this,' Pole says, in an odd voice.
Summers fetches the camera. 'Get out of the way will you, Riley.'
Pole sits back on his haunches, pointing with his trowel. 'He's been shot. Look at the damage to that rib.'
'Arrow?'
'Bullet.'

When he was eleven years old, Jack Riley spent two golden months at Greystone House with Gwendolyn, Ian, Rory and Jamie. Seventeen years later his job as a newspaper reporter brings him back, to learn that it wasn't just the children who kept secrets that summer. Those days just before the war were not as idyllic as they seemed, and instead of writing about the discovery of an Iron Age treasure hoard, Riley finds himself reporting on a murder inquiry to which his own memories may hold the key.