This apartment is superb. It may be quite small for more than 2 people (it was perfect for me), but its location, cleanliness and comfort more than makes up for it. A very short distance from Madrid-Atocha station and a Metro station, with plenty of restaurants in the neighbourhood, yet the apartment is very quiet. It's also got everything for laundry and cooking light meals.
Concepcion is a lovely, helpful and very prompt host. She readily accommodated my late arrival, provided a welcome pack of croissants, and quickly offered to let me use her hair dryer when the one in the apartment broke down.
I will definitely consider this apartment if I revisit Madrid! Thanks Concepcion!