As we follow him across the garden to an L-shape of converted farm buildings, my eyes are drawn to the swathes of spring flowers breaking up the grass and the landscaped pond with its own jetty. Like everything about this house, it’s impressive.

Beyond the garden is another large, open stretch of mown grass where I imagine the helicopter must have landed. The farm buildings themselves have been stylishly converted, the courtyard in front of them heavily gravelled. After unlocking a door, Adam leads us into a room towards the corner of the L. ‘If you look here …’ Showing us a floor plan, he indicates the space he’s measured. ‘You agree that fits with the plan?’

As far as I can see, it looks in order, but then he takes us outside, through the door in the other half of the L, into a similar-sized room. Along one wall, there’s a trestle table and underneath it, several boxes. Opening them, I find unopened bottles of spirits, then in another, unwashed glasses. ‘I’d say someone’s been having a party.’ I turn to Adam. ‘You were saying?’

Showing us his floor plan again, he points to the wall. ‘Even if you look outside, you can see there’s quite a lot of space behind here that isn’t accounted for.’

His words are confirmed by the measurements on the floor plan, but even without them, it’s obvious that something is off. ‘So how do we get in?’

Hurrying outside, I walk around the back of the building, following the narrow path that vanishes into overgrown rhododendron bushes growing up against the wall. Pushing my way through, I find a door into what could quite possibly be the area of space that’s been bricked off, then notice the outline of a path leading off in another direction. My heart starts thumping. There’s no question we’ve found something.

‘This door isn’t marked on the floor plan.’ Adam’s voice comes from behind me.

Mason surely couldn’t have imagined an estate agent would miss this. But then I remember. His plan was to be here, until we interrupted him. Who knows what he was going to say to Adam? Standing back, I stare at what Adam’s holding out to me: his bunch of keys.

I try each in turn, but none of them fit. Then I shove my shoulder against it, but it doesn’t give even slightly. Handing the keys back to Adam, I get out my phone.

The DI answers immediately.

‘Sir? We’ve found a locked room among some converted outbuildings at Mason’s place. The estate agent doesn’t have a key, nor does the space appear on his floor plan.’

‘We need to see what’s inside, May. Can you force it?’

‘We’ve tried. It’s pretty solid, sir.’

‘Right. I’ll get someone over to help. Stand by.’