Then he turned to the piece of equipment that resembled a small fax machine with a glass hood over the top of it. This was the ESDA machine. I had done my homework and knew this was the contraption that had brought down the West Midlands Serious Crime Squad and had exposed a string of other miscarriages of justice when a Birmingham University academic, Tom Davis, had used it to prove that the police had forged confessions. Davis then worked on the murder of PC Blakelock in the Broadwater Farm riots of 1985 and the Birmingham pub bombings of 1974, two of the highest-profile cases of the time. ‘ESDA is an extraordinary machine,’ he had written. ‘It brings to light evidence that was not only hitherto completely invisible, but whose very existence was unknown.’ When I had first heard of this procedure from the private detective, I had joked that in a perfect world the letter-writer would have written her name and address on the page above in the pad.

‘There’s a vacuum pump in the bottom of the equipment that draws the document down through tiny holes and flattens it on to a bed,’ the forensic detective explained. ‘If we cover that document in plastic to protect it, we can charge the document up using a probe with 5,000 volts of static electricity. Now any of the paper nap that has been indented will lift up. So even though physically you can’t see anything on the paper, written in ink, something’s been indented into it. Then we spray the document over the top of the plastic with carbon granules and these adhere themselves to the indentations on the document, should there be any. And then, if we’re very clever, we can lift the plastic off and put it over a white piece of paper, and then Xerox it, so you’ve got a record of the indentations.’ Wonderful.

‘To do that,’ the detective continued, ‘we do need to humidify the document. It’s a very simple process; you have to leave the document in a tray that has water in it, just so that it dampens the paper itself. This lets the electrostatic electricity get hold of the nap of the paper.’

This seemed to me one of my most promising avenues of research – and the detective was infected by my optimism. He was ready to begin. He put on rubber medical gloves and began to slide the letter out of the bottle. I appreciated the little professional touch of the gloves. I noticed that his fingers were trembling slightly and was gratified to see that even a forensic detective was excited by the possibility of cracking the mystery.

The first problem was that the letter was rolled into such a tight scroll only a centimetre or so in diameter. The detective unrolled it, and then unfolded the pages. ‘These large creases could cause a problem for the ESDA machine,’ he commented worriedly, as he struggled to flatten the pages.