Clyde and Rani stayed for tea but eventually she grew tired of them humouring her and she sent them home. Life had to carry on as normal, and after the last week normal was good. She hugged them in turn, wished them sweet dreams in a way that was not lost on any of them.

And then she closed her front door, shutting herself away in the big, empty house on Bannerman Road.

She couldn't sleep much that night, nothing to do with nightmares or the threat of them. She found that she was listening out for the sounds that she now missed. The whirs of K9 above her head as he and Mr Smith exchanged silent jibes. (It hadn't been silent once. Then she'd threatened to use her lipstick in ways that they would never be able to fix.)

Sometimes Luke would get up in the middle of the night for a drink. First time he'd done it she'd snapped awake, not used to there being someone else in her house. Now she wasn't used to being alone in their home and she missed it.

A dreamless-unconscious came in fits and starts and eventually she gave up trying to force it and she got up. Wrapping a gown around her she went downstairs and put the kettle on, leaving the tea bag in the mug to seep for a bit. There was nothing on the TV except signed repeats of soaps and the rolling News 24 so she picked up her laptop and booted it up, removing the tea bag from the mug and adding milk while the welcome chimes echoed through the house.

For a split second she almost dashed to turn it down, but who was there to wake?

She cleared her emails, checked out a few things online, but eventually even she ran out of things to do. She was about to shut it down when she noticed a new icon on her desktop. A video file.

Play Me.

She double clicked on the icon and the video sprung into life, revealing a jolting shot of the attic which was then steadied.

Automatically she smiled as Luke's mop hair came into view. He settled on the sofa and smiled at her.

"Hey Mum," he said, "so I'm guessing it's what? The small hours of the morning? Can't sleep, missing me, house too quiet? I knew this would happen. You have been going on about wondering what you'll do when I'm gone, and given you're packing me off with K9 it's like… you're clearing house or something. And now it's just you in the big house with Mr Smith in the attic and you're lonely."

He looked up, past the camera. "No offence," he said to the Xylok out of shot.

None taken, purred Mr Smith's reply.

"Look, Mum, you had a life before me, right? And I know you told me that things got better because I… y'know," he said, still embarrassed to be saying this to a video camera. "But that's not changed. You're still my mum and you will always be my mum. I know you didn't plan on taking me in but I know you don't regret it. And I know that because I had a nightmare in which you said it was."

Sarah's heart broke a little at this revelation.

"You have taught me so much the last few years and I am so, so grateful. Things could have been so different. I was made to destroy, to take over the world and you turned me around. Everything I am is because of you. I'm going to Uni because of you. Because we were handcuffed by a bomb and you still said I should do what was right for me.

"You helped me to revise even though you barely understood half of it, but you never gave up on me. Not once. You have never given up on me these last few years and I know that when I'm gone you're going to wonder what to do with yourself.

"So this is your guide to how to live a Luke and K9 free life.

"One. It's not Luke and K9 free." Luke waved his mobile in front of the camera. "I know you can use yours so feel free to be the kind of mum who calls her child every day just to check that they're eating right, have clean clothes and are still breathing.

"Two. Unlike you I can cook. And I can work a washing machine. Breathing is pretty instinctive but if it helps I'll have K9 continually monitor me and he can transmit it to Mr Smith.

"Three. I can do this. I can do this because you made me. The Bane put me together but you made me. I am Luke Smith and I am your son. I am who I am because of you. Me, Clyde and Rani saved the world today because of you. We fight back and we defend Earth because of you. Think I can cope with Uni.

"Four. It is forty-nine point two two miles from Ealing to Oxford. Journey time of about an hour. I will come home. You can come up."

Sarah gave a short laugh. "I should think so too."

"Well, it is your house," Luke added after a slight pause as if he'd known what she'd say. "I'm just not around to mess up my room and get the fire extinguisher when you want to cook eggs. But I'm still here.

"Five," he continued, "you travelled in space and time. I'm just up the M40. I need to see what's beyond Ealing and London and you know I need to do this."

Luke shifted off the sofa to his knees so he was closer to the camera. "But the biggest thing, probably the most important thing you need to remember is this. I love you. I love you because you gave me a home and friends and a name. You gave me a family and a purpose and a place in this world. And when I was scared and unsure how I fitted in, when things went wrong because… because I didn't have a childhood or a birthday or even a belly button, you never stopped caring and you never stopped being there.

"You're amazing, Sarah Jane. I couldn't have chosen a better mum even if I'd had a choice. You… I don't know how to put this into words. What you've done for me, what you mean to me. I was scared that you didn't want me, that you just wanted me to go… because I need you. I am always going to need you and I am always going to want you around. Even if it doesn't seem like it.

"I know you're going to worry about me and so it's pointless telling you not to. I'll be home for Christmas, if not before, but before I go there's one thing you need reminding of. You are not on your own. Before you had no friends and, well, half the street avoided you and the rest thought you were weird. You can't retreat back into that because Clyde and Rani won't let you.

"And no matter where I am, whatever dimension my mind ends up in, you are still my mum."

He got up and the video went black. She sat for a moment, staring at the laptop screen before she logged off, picked up her tea, and headed back to bed. Maybe now she'd be able to sleep.