I walk along the river, dazzled, and clasp my hands to my ears like a child as plane after plane screams above me, the river their flight path. The London Eye glints a mile away, on the opposite side, its pods revolving so slowly I cannot see them move.

At the start of the next two-minute lull between the planes, I unblock my number before putting my phone to my ear. The blood is pounding in my head while I wait for Eliza to answer. She doesn’t. It goes to voicemail.

‘Hello, Eliza,’ I say. ‘It’s Helen. Trudy passed on your message. I hope you and Alice are okay. I’m glad you got in touch.’ I give her my number before adding, ‘I can’t believe you don’t have it – I’m so sorry about that. I’m about to go into an appointment, so I’ll have to turn my phone off for a while, but I’ll put it on again as soon as I finish. Bye for now.’

The building rises up like an ugly stone fortress, and I aim myself straight at it. I am startled by how unguarded it seems, with no wall to surround it, and how huge it is in reality, the size of a large block.

The main entrance looms before me. Marble steps pass beneath a massive archway that leads to three sets of double doors. I am stumped. Do I dare walk under the archway? Which set of doors do I try? I really haven’t thought this through. The place where real spies go, and I am clueless.

Two armed police officers glance in my direction. Meanwhile, the statues of Britannia and St George are looking at me from far above, down their noses. A carved face hangs like a portrait, front and centre of the archway’s exterior. The face seems to be watching me too, despite its blindfold, as are a multitude of deliberately visible cameras.

To the right of the archway is another set of double doors. These are more modest than the others. They are brown. They would be brown. Of course they are brown. On both sides of the doors are duplicate plaques. Visitors Entrance, they say. I am an idiot, I think.

Slowly, I approach. I study the doors, searching for a bell, a door knob, some way to push or pull them open. There is nothing. The doors are a blank, despite their elaborate mouldings and beautiful panelling. I raise my hand and knock three times, as firmly as I can, though I am shaky. My fist on the ornate carving results only in dull thuds.

There is a tap on my shoulder and a voice in my ear. ‘Hello there. Can we help you?’

I spin round, and find myself facing the two police officers. ‘Hello,’ I say.