My nitpick is just that. A few little things to tighten the opening.
First, the list of things in her hands, I think "balancing in one hand coffee, breakfast and birdseed." has a better rhythm to it.
I wonder why the dog didn't growl? My dogs always make some kind of noise when an unknown creature moves into their territory. She might drop the birdseed at the noise, too.
Also I would mention the dog before the couch - "a man and a scruffy dog on my expensive new sofa..." maybe a bit of outrage about his dirty shoes on her sofa as well?
"How the hell had he got up here? With a dog? My flat is on top of the building." would be my choice of the next line. Leave out "or so I’d thought. But on reflection," since obviously she thought AND reflected.
"He might be dangerous… a drug addict or a psychopath… a pretty fit one if he climbed up here." I like the "pretty fit" thought. Gives me a better sense of how high the building is, something that isn't mentioned until later. (I think it's four floors, right?)
Lastly, this is long and kind of cumbersome to me:
"I strolled to the far corner to admire a view I never tire of: a London roofscape, a glimpse of the trees in Hoxton Square two streets away, and the distant Gherkin glittering in the early morning sun. Already the faint hum of traffic competed with the coo of a courting pigeon."
How about "I never tire of the view of London roofscape...." to pick up the pace and use the first paragraph to better advantage? Otherwise it loses tension in the description.
I know what comes next, I really like the story. Best wishes, Lexi!