I'm surprised no-one has mentioned this place before...
As a rule of thumb, whenever we are out looking for somewhere new to eat, we always go for the place that looks busy. I have never seen El Veintiseis anything less than packed out at lunchtime, the place is always heaving.
If you look inside, it's a bit drab and could do with a lick of paint. It is noisy, a bit cramped - but those are the only downsides.
The menu del dia is 10 Euros, and for that you get superb basic home-cooked Spanish food, served very quickly by what seems to be an army of waiters. They need them too, they are rushed off their feet - but the point of this place is that it is wildly popular with the locals (you will have difficulty finding a table on a Sunday). Just compare it with the other restaurants nearby, which are half empty even at this time of the year. The owners of El Veintiseis have hit on the secret of success.
Okay, it's not haute cuisine, but if you want a taste of the real Spain, you could do a lot worse. It is now one of our favourite places in the area.