‘Done. The smoke alarm should have gone off by now,’ Glenister said. Houghton shrugged and walked down the corridor to the emergency stairs.

‘Give it a moment. Smoke detectors take a while.’

Immediately, a deafening electronic siren pierced the air. The men waited until doors of the neighbours’ rooms began to open, then they blended with the other bewildered evacuees and streamed towards the fire escapes.

#

On the pavement outside, DS Campbell crouched over the blood-soaked body of a woman. Five minutes earlier, he had been walking with the crowd towards Trafalgar Square, watching the protestors.

His radio crackled. ‘Suspects may be in a building on Pall Mall,’ the voice said.

He waited for the radio to clear and then called in.

‘Many people down, request ambulances urgently. I’m with a female aged around thirty. Wounds to the stomach and face, still breathing. In the centre of Pall Mall. Other casualties close by. Over.’

Campbell looked up. He was exposed, kneeling here alongside the discarded placards and banners and smoke, surrounded by fragments of glass. A petrol bomb, he thought. And where was Skeffington? He’d been with him moments before.

Forty metres away, two colleagues were kneeling next to another body on the pavement. There was a third close by. A cloud of acrid smoke floated his way. Campbell held his hand up to his mouth and coughed. As the smoke cleared, he saw four paramedics running his way. Two peeled away to the other victim, two bounded towards him with a stretcher.

‘We need to shift her now,’ he barked at the paramedics. ‘Get her out of the road.’

He stood back to let them work on the woman and moved to the pavement. No point standing here now, he thought.

A woman with a child came up to the steps of the hotel.

‘Sorry, madam, but you can’t come in here.’

‘But my daughter needs a wee. She’s desperate.’ The woman had a posh Home Counties accent.

‘I’m sorry, madam, you need to move away for your own safety.’

The woman swore and moved on. Then the fire alarm went off, and within seconds, people started to flood out of the front doors.

#

CTOC, St Pancras, London

‘Update, please,’ said Gabriel.

The DS said: ‘It’s likely the attackers were in a hotel overlooking the route. The A-Line in Pall Mall. Someone set off the fire alarm – unsure whether it’s a genuine fire as of yet. We now have snipers on three roofs overlooking the front and rear of the hotel. They’ve clear vision of the exits, but there’s a lot of people leaving the building.’

‘And the crowd?’

‘We’re struggling to contain the rally inside Trafalgar Square, ma’am. They are getting panicky. Looks like we’ve got three dead, a dozen injured, maybe more.’

‘Get someone in the hotel,’ said Gabriel. ‘We need a list of residents and we need to look at the CCTV. These places are crawling with cameras.’

#

Pall Mall, London

Outside the hotel, Campbell’s radio crackled. ‘Suspects believed to be in A-Line hotel. Armed. Armed units make your way to A-Line hotel, Pall Mall, immediately.’ He forced his way through the bodies fleeing the hotel and into the reception.