We (3 adults) spent a great time at La Latina. It is situated perfectly, less than 10 minutes from the Plaza mayor, 4 minutes from the metro station Tirso de molina, where you also can find a carrefour supermarket 24/7.
Lisi and Marta were great hosts, always available, and they gave us some very good ideas of what to do, where to eat or where to book excursions.
There are some very good places to eat nearby for people who suffer from the celiac disease; you find them on "https://celiacos madrid.org" - comer sin gluten - establecimientos.
About 10 minutes from the flat they rent lockers to leave your suitcase for a day (http://madridlockers.com/consignas/lockers-plaza-san-miguel-2/), in case you arrive too early or leave late.
The apartment ist well equipped, furnished with very nice details, spacious, has a very good heating (and I guess air conditioner as well) system, a balcony and rolling shutters in the 3 bedrooms. There was shampoo, shower gel, conditioner, toilet paper, hair dryer, towels, water boiler, coffee machine (Nespresso), kitchen towels and paper, oil, vinegar, spices, salt, sugar...
We highly recommend this place!