The engineer unlocked the door into a final stretch of corridor. There were no explosives, but like most buildings built before the 1980s, the mineral asbestos was used as fireproofing. Now known to cause cancer, every sheet of asbestos had to be removed so that its toxic dust didn’t form part of the cloud when the building detonated. This hallway had been the last to be cleared, and the plaster and ceiling had been stripped back to bare concrete, with some dangerous-looking 1970s electrical wiring on show.

James felt intimidated when he stepped into a room which he realised sat directly beneath the campus dining-hall. There were two dozen rows of metal shelving, each stretching more than thirty metres. Everything had been cleared out, apart from the five hundred and fifty dusty box files, each one recently stickered with Mission Control – do not destroy.

Made from heavy card, each box was more than a metre deep, and James quickly realised that it would take two people to lift each one upstairs.

‘OK,’ James said, as his sister and several kids groaned at the magnitude of the task ahead. ‘It looks like we’ve got our work cut out.’

Four hours later, James had backache and pit stains the size of dinner plates. The fourth and final truckload of boxes had been shifted and the last batch of exhausted kids were clattering upstairs with the metal trolleys.

‘They worked hard,’ Lauren said, giving James a smile. ‘You’re actually really good with the kids.’

‘You think?’ James said brightly. ‘I always feel like a phoney.’

Lauren inspected dirty hands and a cracked thumbnail as she set off upstairs a few steps behind her brother. ‘I’ll never be a lady with these hands,’ she said.

‘Weird to think it’ll all be under rubble in a few hours,’ James said, when he reached the ground floor.

He should have headed on down the corridor, but there was no sign of the engineers and just a strand of do not enter tape keeping him from going on upwards.

‘One last look?’ Lauren asked, making James crack a huge smile because he was a quarter second behind in saying the same three words.

James straddled and Lauren ducked under. After furtive glances behind, the siblings moved quickly but quietly, tired from humping boxes but able to handle pain after years of CHERUB training.

‘Saturday night hall parties!’ James remembered, as he rounded the sixth-floor landing.

Lauren smiled and added, ‘Pop tarts and instant mac and cheese in the mini-kitchen.’

With no glazing, human smells and central heating had been replaced by crisp outdoor air that made James feel like he was in a dream.

‘Creepy as,’ Lauren said, as the cold air caught skin damp from hard work.