There’s no way I can get a blind father and a barely conscious man out of an underground reservoir in the pitch dark on my own. The thought is laughable. Terrifying. We need paramedics. Stretchers. We need serious help here. But I feel frozen; I am paralysed and panicked with fear. I’ve also lost my bearings. I have no idea how to rescue any of this. ‘OK, I don’t even know which direction to go in now, Dad. I know you’re somewhere to the left of me, but I’m scared I’m going to set off in the wrong direction – the wrong angle. I can’t go stumbling about. I don’t want to walk slap bang into a pillar or something!’

‘The wall,’ calls Dad. ‘The end wall is behind you. Find it.’

I turn and flail around with my arms. ‘I can’t feel anything! I can’t find it!’

‘Stay calm. My voice is to your left so the wall must be behind you. So, turn around and take six steps in every direction, like you’re travelling to numbers ten o’clock to two o’clock on a clock face, there and back, there and back, and you’ll find it.’

I gingerly turn and take six steps to an imaginary ten o’clock, holding my hands out in front of me; an un-spooky ghost. There is nothing. I retrace my steps and try again to eleven o’clock. Twelve. I don’t think I’m anywhere near that wall. At one o’clock I feel something.

‘Found it,’ I cry. I smooth my hands back and forth on the brickwork. ‘At least I think so. I don’t think it’s a pillar. It’s wider than that.’

‘Can you feel raised letters?’ says Dad. ‘The letters are raised as the paint is thick.’

I smooth my hand over the brickwork again. I can feel a two-inch wide column that’s slightly raised; it curves round into what could be an ‘O’. ‘Yes, I think so.’

‘Then you know exactly where you are,’ says Dad. ‘The letters say “ELWW Hornsey Wood Reservoir, 1868 and 69”. Put your back against the wall.’

I do so. It feels cool and slick through my soaked T-shirt.