They walked quickly away from the scene arm in arm and went for dinner at a restaurant near the Millennium Bridge. As usual Rosa was able to move her thoughts on from the murder she had just committed. However, she was still hyper from the rush of adrenalin and it took two vodkas to ease the tension in her bones.

But after that she was even more relaxed than before because she was no longer under pressure to seek out a victim. The job was done and she didn’t have to look for another target until tomorrow, which meant she could focus all her attention on Alice.

The place they were in was already adorned with Christmas decorations, and there was even a large, brightly lit Christmas tree in one corner. It prompted Alice to ask Rosa where she would be spending Christmas.

Rosa simply said that she hadn’t decided. Then, to deflect any follow-up questions, she launched into a spiel about how Mexicans celebrated the festive season.

‘In my country Christmas is called Las Posadas,’ she explained. ‘We stage lots of festivals, and the holiday runs from December the twelfth to January the sixteenth. And, of course, the weather is warm. Not like here.’

Rosa hadn’t celebrated Christmas for many years. Even though she’d been born into a religious family, the significance of it meant nothing to her. Three years ago she executed a journalist on Christmas day because Carlos Cruz did not like an article he’d written about the Sinaloa cartel. The man was taking part in a procession at the time with his wife and two children when she rode past on her motorbike and shot him in the head.

‘It’s a pity you can’t be here for Christmas,’ Alice was saying. ‘We could have a great time together.’

Rosa felt a tingle of excitement. ‘Maybe I can be here,’ she said. ‘If you really want me to be.’

They’d been exchanging teasing remarks like this throughout the day. Trying to gauge each other’s reaction. Fishing for compliments or a sign that they were both hoping the relationship would continue beyond the end of next week.

‘Of course I’d like you to be here,’ Alice said. ‘Or if you can’t then perhaps we can meet somewhere in between. I can get time off work. No problem.’

They talked about it as they left the restaurant and walked arm in arm back along the South Bank towards Westminster Bridge.

It was a beautiful evening, full of people and colour. Above the city the dark sky was crowded with needles of frozen light.