So the first thing to note is that although we booked the Plaza Espana IV apartment, we didn't end up staying there...
On arrival at the apartment block we were met on time and let into the building. On trying the keys in the lock for apartment IV, it soon became clear to us and the agent that they were not the right ones. Cutting a long story short, we were ushered to apartment VI in the same building, to which the keys matched, and that's where we actually stayed.
No real explanation was given as to why we were placed in a different apartment, but as it was very similar, we decided to accept it. Whilst we had no major complaints, it was on a much lower floor, and thus suffered more from noise. This was especially true at 3am each morning when the builder's skip was collected from the next door property which was undergoing improvements!! No mention of this had been made to us at the time of booking, or when we checked in. This is really not very good.
Although the apartment was fine for our purposes, and is very well located for the centre of Madrid, and within 5 minutes walk of the Metro, we noted a few items missing compared to the web listing i.e. no oven (only a hob), no coffee machine and no toaster. Although these were not that important to us, we reported them so the listing could be corrected. We did not receive a reply then or since.
Some other small niggles ... the lock on the building front door was difficult to open with the supplied key, and the shower drain in the bathroom was partially blocked. Again, we reported these but no reply has been received.
None of the above spoiled our holiday in Madrid though, and we thoroughly enjoyed ourselves. The visit to the Royal Palace and day trip to Toledo were great.
In summary, the apartment we actually stayed in was just fine for us, but some attention to better organisation and communication would seem to be needed.