It all goes swimmingly until we get to Carlisle. I am no more than eight miles from the Scottish border when it begins to go wrong.

We have not long pulled out of Oxenholme. I have been staring out of the window at the Lake District wishing that instead of being stuck on a train, I was on foot in a wilderness – wouldn’t fleeing that way be much more exciting? Be careful what you wish for, as they say.

We have come to a stop some way out of town, far enough out for fields to stretch either side of the train. The sky is still pale blue but if I look ahead on the left-hand side I can see clouds, bunched and darkening, massing together like an infantry unit preparing for attack. I shuffle to the aisle seat so I can look down the train and see a ticket inspector in the carriage ahead. He is beaming at passengers while he asks for All tickets and railcards, please and bumping against the side of the seats as though the train is still in motion, laughing at his own joke. I have a valid ticket, bought six weeks ago, but he’s the type who pauses to chat to customers, which makes me nervous. I rise, pick up the holdall, and start walking down the train to the rear carriage. As I do, the train begins to move and for the briefest of moments I have the illusion that although we are picking up speed, I am stationary.

In the rear carriage, I spot a set of seats around a table at the far end that look empty. It is only as I sit down, a little heavily, that I see there is a woman in her thirties with an infant on her lap sitting in the seat diagonally opposite mine, facing backwards. Next to her is a child around six, leaning against her mother’s shoulder and sucking her thumb.

Startled by my sudden arrival, the woman glares at me and I see that on one cheek there is a bruise, only partially disguised by powdery foundation. It is in a shallow crescent shape beneath one eye and already a purple-greenish colour, with a glow of yellow that blurs the edges, as light can sometimes blur the edge of the moon. I bet that hurt before it discoloured, I think. Now the swelling has gone down the pain will have diminished, perhaps so much that she feels a flush of surprise when she looks in the mirror. Or, perhaps, she might be so accustomed to bruises she never really forgets they are there. If you are beaten a lot, then your safety depends on you never forgetting.

Our eyes meet and in the same instant I observe what is all too obvious in her – the bruise, her hidden position on the train, the quiet, glazed look of the child – she sees something in me, and I almost want to lean forward and say quietly, It isn’t what you think. My situation is different. That fellow feeling you’ve just spotted, it’s not the same.

She gives me a cool look, then turns her face to the window. I don’t know whether I am more embarrassed by my seeing her or her seeing me, but I rise from my seat again, pick up my holdall and head into the vestibule at the very rear of the train, an empty space where nobody will come. I have an overwhelming desire to act.

I take my burner phone from my coat pocket. I turn it on and dial as the train picks up speed again and soon we are thundering towards Carlisle.

It answers after four rings. ‘Stuart,’ I say, ‘Sophie Lester. We spoke in May.’

There is a short pause. ‘Ah yes, Sophie, yes of course, of course!’

I am just in time, he says. End of September is my last opportunity. After that, the weather will be too bad to sail.

It’s late August – the end of September leaves me with four weeks to go and in an ideal world, I’d be out of the country within days.

‘You don’t have anything before then?’

‘Sorry, next one is full and I can only do so many, only so many people that crazy!’ He bursts into one of those hahahaha laughs.

I make a calculation – I’ve always known I would have to go to ground for a few days at the very least and I have the wherewithal. This isn’t catastrophic, just a hold-up. I consider offering to bribe Stuart to toss one of the earlier participants off his manifest, but it might make him suspicious and that I cannot afford. ‘Sure, just as long as you’re definitely going then.’

‘Oh yeah, I have to, still paying off the loan! Hahahaha!’

After a few unavoidable pleasantries, I bid Stuart farewell, hang up and consider my plans. Four weeks. I will have to divert my route, take my time over each stop – a zigzag course would be good, that’s okay. I’ve studied my Ordnance Survey maps of Scotland so often over the last few weeks that I’ve practically memorised the whole country. It will actually play in my favour: it will add a note of randomness to what I do next.

I should message Vikram, I think, using the code we agreed in advance.

It is at this point, as I’m standing in the rear vestibule of the train as it approaches Carlisle, about to text my solicitor, that something really bad happens – so bad that I have to go through a series of thoughts in order to calibrate just how bad it is.

The phone in my hand vibrates.

The only phone call I have ever made on this phone is to Stuart the yacht guy, just now, and I withheld my number. Nobody has this number. It is an unknown caller.

I lift the phone to my ear.

‘Heather …’ His voice is low, almost a murmur. ‘Phones on the London train, bit obvious. Doubt that’s where you’re headed.’

I say nothing. I wonder where he is, whether he’s back in his corner office, or whether he’s still standing in Alaska, having dismissed everyone else, looking out over Birmingham.

‘It’s okay …’ he says. Is there a slightly breathy quality to his tone? I am trying to work out if he sounds frightened. ‘They already know you’re not going to London, so this is what …’

I hang up the phone and turn it off. I stand where I am for a moment or two, biting the inside of my cheek and confronting the possibility that I have not been nearly as clever as I thought.

He didn’t say we. He said they. This is – potentially – very bad news.

The train pulls into Carlisle. I get off, shouldering the holdall. I don’t glance into the train but as I pass the woman with the two children, I have the feeling she is looking at me.

Carlisle Station is unexpectedly pretty – a vaulted glass roof, ironwork, stone. This calms me a little as I head for the exit, walking ordinarily, so as not to attract attention of any sort. As I reach the exit barriers, two men in slacks, one wearing a herringbone coat, the other a mac, come walking swiftly towards me, their faces set. I keep my expression neutral as I show my ticket to the guard while watching the men out of the corner of my eye. There is another guard at the far end of the barriers, next to an open gate, and they flash warrant cards at him, discreetly, without pausing their stride. I can’t risk looking back so I don’t know if they are heading towards the train I have just left or whether they are here about something else. I need to stop somewhere and regroup, but I want to be further away from the station.

I stride off in what looks like the direction of town, passing a church and two solid round towers the shape of the castle pieces we used to have in an old wooden chess set. I don’t stop as I go by but glimpse a plaque that says something about a citadel and think, you’ve built your citadel, it’s going to be okay. So Kieron was already tracking your phones – you suspected as much. The burner phone is too old for GPS but him having the number is a deeply alarming development.

I head down a road called English Street, which strikes me as mocking when, if I’d stayed on the train, I could be nearly at the Scottish border. Perhaps I’ve been precipitate – or perhaps the train is still sitting in Carlisle Station while those plainclothes officers walk up and down the carriages looking for me.

I am walking through a pedestrian precinct and see that at the end of it there is a Victorian-looking building with wide steps leading up to a Tourist Information Centre and beneath it, a café. I wonder if I can risk going in to pick up a local map and decide that I can’t – but I can sit in the café, which is wide and open plan, with seats inside and out, and have a clear view right down the precinct.

He didn’t say we, he said they.

We call it sweeping the room.

One of the visiting speakers on our surveillance training course was seventy-eight years old – such a long string bean of a man, pale and bony, white-haired, manner as cool as a glass of water – he came to talk to us in jeans. Jeans on a septuagenarian, we loved it. His tone of voice had a mid-Atlantic inflection. Although he was elderly, kindly, there was something sexually alert about him, a slight crackle in the room, like a radio signal from a planet in a distant galaxy.

He was there that morning to give a PowerPoint presentation about...