‘Look,’ the constable interrupted her.

As she pointed at the screen, a group of people appeared, crossing a car park on foot, led by a tall thin man. Just before they moved off the edge of the screen, they caught up with David and his wife who turned to face them. David looked somehow puffed up and angry, while Anne was clearly frightened, her shoulders hunched and her eyes darting around nervously. She tried to hustle her husband towards the car but he stopped and turned to face the people who were pursuing him. The camera didn’t pick up any of the words that were exchanged, but it was clear they were shouting at one another. The tall man shook his fist at David in a threatening manner before they all moved out of sight of the camera.

‘His image isn’t very clear here, but we managed to pick him up on his way into the hall. It’s definitely the same man. His clothes, his gait, all identical.’

The film froze, showing a gaunt face gazing directly at the camera.

‘It’s a good resolution,’ the constable said. ‘We had to enhance it, but you’ve got to admit it’s pretty clear. You could pick him out of a crowd.’

‘Now all we’ve got to do is find out who he is,’ Geraldine said.

The constable smiled. ‘Our image recognition software has found a match.’

She pulled up another screen. Geraldine leaned closer to read the details: Jonathan Edwards, forty-two-years old, divorced, school librarian.

‘He’s a bit aggressive for a school librarian.’

‘A former school librarian,’ the constable corrected her. ‘He lost his job last year. These details need to be updated.’

Eileen was more interested in the identity of the heckler than Geraldine thought his actions warranted.

‘We know his anger’s personal,’ Eileen said, with something approaching glee. ‘He may blame David for the loss of his job and he’s following him around heckling his speeches. Could he be our killer?’

Geraldine frowned, wondering how a stranger could have introduced cetirizine into the dead man’s diet. It seemed unlikely.

‘All he did was shout out at a few public meetings,’ she said. ‘How was he supposed to have persuaded David to swallow pills?’

Eileen frowned. ‘Let’s speak to him,’ she said shortly.

Jonathan Edwards rented a ground-floor bedsit in a converted house off Holgate Road. It was ten in the morning when Geraldine rang the bell and she wasn’t sure she would find him at home, but after she had been waiting on the doorstep for a few moments a man’s voice called out, asking who was there.

‘Is that Jonathan Edwards?’

‘Who wants to know?’ he replied.