I was allowed upstairs and shown into an office. Two walls were covered by audio equipment, banks of tape machines, and other mysterious technical matériel, dials, lights and switches. It looked like a set designer’s idea of the heart of a network designed to take over the world. The forensic detective was clean-cut and tall, with something of the ‘short back and sides’ of the military man. As he made us coffee, he filled me in on his background. He told me that he had spent sixteen years as an electronics engineer in the Royal Navy, which explained his brisk manner, mostly in submarines listening in to various atmospheric conversations, echoes of secrets being passed from potential enemy to enemy. Then he had joined the Metropolitan Police where his skills at computing and electronic eavesdropping had led him to the top of the Forensic Audio Laboratory, which was just coming into its own at the time. He had been called upon to assist with frontline work on authentication and enhancement at several terrorist trials, work that involved providing urgent answers on legal cases in response to lawyers’ demands and then defending his findings before an aggressive barrister. From there he had set up his own company in the commercial sector, although his main client remained the police service, which probably explained the security guard’s question. The company undertook forensics work, computer retrieval, information technology, video and audio enhancement, analysis and transcription of video and audio tapes, and had recently developed a 3D crime scene modelling unit. Although the detective himself specialized in audio rather than documentary evidence, he was knowledgeable about the use of equipment to analyse paper and writing. He had agreed to see me – and the letter – out of pure intellectual curiosity which, given the state of my finances, was more than helpful. ‘If necessary,’ he said, ‘we can get one of the specialists in.’

Before I unpacked the bottle, he told me he would show me his favourite toys. I was impressed by his discipline and refusal to look at the letter until he had explained things to our mutual satisfaction. First he showed me an ACO tester. It looked a little like a computer screen, but hollowed out, like a hood, which shone light on the document beneath it. The detective explained that ACO was just the trade name, that the equipment was a form of vectoral comparator, but more compact. It felt more and more like a scene from a James Bond film, with the detective in the role of Q.

‘It is capable of looking at indentations on paper,’ the forensic detective continued, ‘but it is generally used to look at inks. The idea is that each individual ink will fluoresce at a different light, even though the manufacturer may be the same, so when you’ve got two black inks, they will fluoresce at slightly different wavelengths.’ He slid a test card, a bogus cheque from ‘Nicky Smith’, under the hood to demonstrate what he meant. Then he adjusted the type of light and the frequency, did something to the filter and twiddled the focus button until the inks began to separate out. It became clear that Ms Smith had written a cheque for £6 and some fraudster had increased it to £60, but though the forgery was indistinguishable to the naked eye, it was clear under the ACO tester.