Liam reads the description below the photo. ‘“Andrew and Nancy McKay with their son, Elliot”. Yeah, looks like they were the last owners. The book was published seven years ago.’

I study the picture. Andrew, who looks to be in his forties, has a long face with piercing eyes. He’s tall and rangy and has a sullen expression on his face as he stares directly into the camera, like he’s staring down an enemy.

His wife, Nancy, looks to be in her mid-thirties, beautiful, with a watchful gaze. She’s slim and delicate looking, but perhaps that’s only emphasised because she’s standing beside Andrew.

Elliot looks like his mother: slight, with light-coloured hair, though it’s hard to tell the exact colour because it’s a black-and-white photo. He’s clearly pale though, and I can just about make out freckles dotted across his nose and cheeks. A large Greyhound dog stands beside him and his hand is resting on the dog’s back. Similarly, I notice Nancy has her hand on her son’s shoulder. Meanwhile, Andrew is touching neither his wife nor his son, nor the dog, but stands apart, hands hanging at his sides, curled into fists.

Liam shuts the history book and picks up a map of the island. It’s old and creased and he spreads it out over both our laps. He stabs his finger down where the cottage is marked. ‘This is us,’ he says. ‘We’re at the south-east end of the island.’

I glance at the key at the bottom of the map and calculate that the island is approximately two miles long and about a mile-and-a-half wide. The castle is marked right in the centre, with the forest to the east and the cliffs rising to the west.

‘The book said there’s an ancient barrow somewhere,’ he says, sounding excited.

I shake my head, confused. ‘What’s a barrow?’

‘It’s where the Druids and the Picts used to bury their dead,’ he explains eagerly. ‘A stone and earth chamber. I just read about it in the book. I thought we could go looking for it.’

I remember what the pub landlord said about the ancient burial sites. ‘OK,’ I say, trying to summon some enthusiasm. I know Liam is a history buff so I’ll have to let him drag me around in search of this barrow. ‘Does it give any indication of where it is?’ I ask.

He shakes his head. ‘No, but we’ll find it. I am a detective after all.’

I glance at the map. The forest seems to cover about two thirds of the island, with grazing land marked on the far western end, where the cliffs are. It’s a large area to scour for what is essentially a mound of earth.