A great little flat in a nice part of Madrid. The flat was very convenient for the train station, the art museums and also the centre of Town. I did not meet Sergio but the check-in process was painless, he hid the keys for us to collect. The flat itself was fine for our needs, though I would echo other comments about the Internet being intermittent and slow at best and that the location of the flat at the top of the block may make it unsuitable for anyone with a large amount of luggage.
Overall I was very happy with the apartment and would definitely consider it the next time I visit Madrid.