Most of all he was beginning to develop a nagging doubt about Gallagher. He had never been convinced that there was enough evidence to link him to the death of Clark. In any event, he could not work out any real motive that Gallagher might have for wanting the death of his father’s best henchman. The Boyle case ought to be strong enough to secure a conviction. At least there was evidence and motive, and that combination was never a bad start.

Arguably the best evidence was yet to come: one more eyewitness, who on the face of it should be independent and, hopefully, untainted by outside interference. The forensic evidence should stand up. Then, of course, the confessions. The confessions!

He could accept that in an unguarded moment Gallagher might have said something to a police officer which he would later regret. But Warwick? Now, this was very different.

There is a popular myth that a code of silence is operated by the criminal classes. Honour among thieves. Omerta. Not so. Criminals will readily provide the police with information against their best friend in an attempt to secure a favour from the officer concerned or to do the dirty on a professional rival. Informers are usually secretive, nocturnal creatures. This is one business in which it does not pay to advertise. The subjects of their reports do not take kindly to having their activities discussed. Frequently they express their disapproval in clear and painful terms.

Arnold Warwick was about to enter the witness box and grass in the most spectacular manner. He would know that the Gallaghers would never forgive him and make it their business to exact appropriate revenge. Why this man was taking such a risk was an unknown factor. He himself was awaiting trial on a charge of armed robbery. Initially he had offered to give a statement if this charge was dropped. This offer was declined. Then he indicated he was prepared to speak if the judge at his trial was told of his co-operation. This was acceptable. Warwick agreed to testify even although he was aware that there was no guarantee the hoped-for benefit would live up to expectations.

McAllister glanced at his watch. Taylor was late. Not like him at all. He continued staring at the notepad. He looked up when he heard footsteps hurrying towards his office. Tom Donaldson, the Productions Officer, hurried in. The look on his face sent an involuntary shiver down McAllister’s spine.

‘Boss, there’s been an accident at Hillhead underground station. It’s Bob Taylor. Boss, he’s dead.’

The impact of this news struck him like a punch in the solar plexus. Before he could gather his thoughts or utter any comment, the ringing of the telephone made him reach instinctively for it.

‘Superintendent McAllister?’

‘Yes.’

‘It’s James Muirhead. I think we had better meet. I have something to tell you.’