‘You should check the doorknobs for Mutti’s fingerprints too,’ I said, swallowing hard and trying to ignore the tremor in my voice. ‘And you should probably check her toothbrush while you’re at it.’

Pinstripe made a little noise then that might have been a suppressed laugh. He was examining the speaking tube with interest. I forced another breath in through my tightening throat. As long as he was looking at the speaking tube, he wasn’t looking at the drawings.

‘That tube goes down to the kitchen so you can talk to whoever is in there and ask them to bring you up a sandwich or something.’ I was beginning to gabble now. As I spoke, my eyes flashed over to the pile of sketches. They were on a seat just a few paces away. The policemen still hadn’t seen them. ‘You talk into the tube and then put it to your ear to listen to the reply,’ I went on. ‘That brass piece is a whistle. If you attach it, people at the other end can get your attention by blowing into the tube and sounding the whistle.’ I don’t know why I was telling him so much. Once I had started, I couldn’t really stop. But there was also something about the way Pinstripe listened – tilting his head slightly to one side like a robin – that made me keep going. ‘Mags and I used to use it to eavesdrop on whoever was talking in the kitchen, until Mutti told us it was bad manners. If you listen very carefully, you can usually pick up the conversation, even if people aren’t speaking directly into the tube.’ Pinstripe raised his eyebrows. He put the speaking tube to one ear.

‘All quiet in the kitchen now,’ he said softly.

I pictured Pa, all alone, pale and sunken in his chair.

‘Is it possible,’ hissed the first policeman, ‘that the suspect could have used this device for listening in on Mr Smith’s conversations with the Admiralty?’ He slicked down his oily moustache with a finger and looked very pleased with himself.

There was a pause. The second policeman nodded, impressed, and started to write something down in a notebook he fished out of his pocket.

‘No,’ I said, as firmly as I could manage. ‘The telephone is in the service room on the floor below.’ Then, to make them feel even more stupid: ‘You walked right past it on your way up the stairs.’

The oily policeman smiled at me – a sarcastic, fleeting little smile. His moustache twitched. He put on a pair of gloves, and started turning the pages of Pa’s logbook – the official record of lighthouse activity and various weather and shipping charts. He showed it to Pinstripe. The second policeman was poking around in a cupboard full of tools and oil cans.