She licked her lips. Perjury starts with small beginnings.

‘That’s right,’ she whispered.

She submitted to the cheek swab and gave various details about her address and contact numbers. ‘I’m sorry to have to ask this,’ Wickens said, finally. ‘But may we borrow your mobile phone for half an hour?’

‘Why do you need it?’

‘It’s actually to help you,’ Wickens said, with a smile. ‘We’ll use cell site analysis to prove your location. It just helps us be sure that you have been telling the truth.’

‘But you only need the number for that. You want to look at my messages and emails as well, don’t you?’

‘No, of course not,’ Cottesloe said. ‘But it would make things faster.’

Leticia looked from one to the other. Anton hadn’t mentioned having to hand over his phone, so she didn’t understand why they would pick on her. She brought out her phone, and then realised that she couldn’t hand it over. She had exchanged a couple of emails with Jill Allsop about Neville Rollason. She was pretty certain that those emails didn’t include any of the confidential details, but she was going to try something.

‘I’m sorry, I can’t give you my phone.’

‘That’s very unfortunate, Ms Mountjoy,’ Cottesloe said. ‘It doesn’t look good does it? What have you got to hide?’

Leticia delved into her handbag, and finally found the business card that DI Morgan had given her. ‘Ring him first, he’ll explain.’ She hoped that she had done the right thing. The card mentioned his rank but made no mention of the fact that he was Special Branch.

The two police officers looked at each other and, excusing themselves, walked out of the room.

‘She must be a snitch, and Morgan is her handler,’ Cottesloe said, as he took out his phone.

‘That makes me more suspicious, not less,’ Wickens said. ‘I don’t know what she could be doing that is more important than us investigating a dead body.’

Cottesloe made the call and got straight through to Morgan. ‘Sir, sorry to bother but can I just ask whether you are running an informer called Leticia Mountjoy?’

Morgan seemed confused for a minute. ‘The name’s familiar, hang on. What’s it about?’

Cottesloe explained where she had been seen. ‘She may be a witness to a death, you know that body in the Thames, but she doesn’t want to hand over her phone.’

‘I remember now. I’m sorry, Constable, you have to lay off her. Let her keep her phone.’

Cottesloe tried to suppress his groan of disappointment. ‘Sir, may I ask what your interest is in her? It’s just that it may impede our investigation.’