It was not until we went to examine the jump that the problem became apparent. Put simply, we had massively underestimated the distance between the buildings. Francis’s calculations from the ground had figured on a running jump of about six feet on to a flat roof, with perhaps a six-inch difference in height between one rooftop and the other. For him, the mistake was so uncharacteristic, and the look in his eyes was strange when we saw the truth of the gap. The leap was at least eight feet wide, and though it was possible to sprint at it, the white stone balustrade that decorated the roof of the Senate House would interrupt the run, and widen the gap by a further foot. However, if negotiated properly and at speed, it would also provide another two feet of elevation. For the first person across, there would only be one chance. The leap would mean success or death. Even for those of us following behind with the benefit of the safety rope, a fall would dash our soft heads against the stern walls of Caius.

We debated the point a little, but only Francis had any real enthusiasm for continuing. The rest of us entertained the notion, but did so mostly as a sop to our own sense of pride, so that we would not seem to have given up too easily when we found ourselves warm and safe in our beds.

Then there was the sound of a car pulling up in the street below. We were huddled together in the centre of the roof, and could not see over the edges, but we did not have to glimpse the vehicle to know it was a police car. The sirens were quiet, but the lights swung blue black, blue black, casting an eerie glow on the surrounding brick and stone. For a few seconds we held our breath, hoping they did not know we were there. Then we heard the sturdy clunk of the doors, and a voice called out through a loudhailer: ‘You there, on the roof. We know you’re up there. The building is surrounded; we’re putting a ladder up to you. Please stay where you are until we can get you down safely.’

We looked at one another. Francis silently pulled up the rope at the side of the building. It had been sloppy to leave it there, but no one reproached him. It was not the time.

‘This puts a rather different complexion on the matter,’ Michael said. At that moment his pompous bravado was rather wonderful. If we could still be ourselves in these circumstances, I thought, they must not be so desperate.

‘Do they think we’re robbers?’ I asked. ‘Will they believe us if we tell them the truth?’

It was a childish question, and the others ignored me. Instinctively, they turned to Francis.

‘We must do it!’ he said, his eyes shining. For a moment, in the madness of adrenalin, it occurred to me that he himself might have summoned the police to provide the final spur to our jump.