The flat is beautiful, just like the pictures. It's a short ~8 min walk to a gorgeous sandy beach, and about as far in the opposite direction is high street with restaurants and whatnot. The nearest subway station is ~5 min away, and I found that the underground gets you pretty much anywhere in Barcelona :)
Having a little corner shop in the building next was convenient. You'll get access to the roof (~4th floor or so), which was not as exciting in December as it would be in summer, I imagine, but still pretty cool. It's a pleasant 30 min walk away from Sagrada Familia.
It did get a bit cold in the evenings, but I hear that's the norm for Barcelona housing in winter. Aircon and UV heaters did not help very much, so I got fluffy socks and a cosy jumper and was sorted.
Thanks Jane for being so responsive, Javier for picking me and my friend at the airport, and helpfully giving her a lift to the place she was staying at. Special thanks for re-scheduling key drop-odd an hour earlier (on a Saturday!) at the last minute on my request :)