Dr Elekton said: ‘What’s all this about the stanchion?’

‘We’re wondering if it was used as a lever for the drain-pipe. We’re not very likely to find anything on the pipe itself after the rough handling it’s been given, but it’s worth trying.’

He walked to the end of the drain, returned on the far side to the solitary pipe and squatted beside it. Presently he said: ‘There are marks – scrapings – same distance apart, at a guess. I think we’ll find they fill the bill.’

When he rejoined the others, he stood for a moment and surveyed the scene. A capful of wind blew down Green Lane, snatched at a corner of the tarpaulin and caused it to ripple very slightly as if Mr Cartell had stirred. Fox attended to it, tucking it under, with a macabre suggestion of cosiness.

Alleyn said: ‘If ever it behoved us to keep open minds about a case it behoves us to do so over this one. My reading so far, may be worth damn all, but such as it is I’ll make you a present of it. On the surface appearance, it looks to me as though this was a premeditated job and was carried out with the minimum of fancy work. Some time before Cartell tried to cross it, the plank bridge was pulled towards the road side of the drain until the farther ends rested on the extreme edge. The person who did this, then put out the light in the lantern and hid: very likely by lying down on the hard surface alongside one of the pipes. The victim came out with his dog on a leash. He stepped on the bridge which collapsed. He was struck in the face by a plank and stunned. The leash bit into his right hand before it was jerked free. The dog jumped the drain, possibly got itself mixed up with the iron stanchion and, if so, probably dislodged the lantern which fell into the drain. The concealed person came back, used the stanchion as a lever and rolled the drain-pipe into the drain. It fell fourteen feet on his victim and killed him. Ha – hallo! What’s that!’

He leaned forward, peering into the ditch: ‘This looks like something,’ he sighed. ‘Down, I fear, into the depths I go.’

‘I will, sir,’ Fox offered.

‘You keep your great boots out of this,’ Alleyn rejoined cheerfully.

He placed the foot of the steel ladder near the place where the body was found and climbed down it. The drain sweated dank water and smelt sour and disgusting. From where he stood, on the bottom rung, he pulled out his flashlight.

From above they saw him stoop and reach under the plank that rested against the wall.

When he came up he carried something wrapped in his handkerchief. He knelt and laid his improvised parcel on the ground.

‘Look at this,’ he said, and they gathered about him.

He unfolded his handkerchief.