Hi there, do you think that the Pixies track might have been Vamos instead of Isla De Encanta? That's what is on the Happy Otter mixtape which otherwise surrounds other tracks from that show, and it's also from their album from a couple of years previously making it unlikely he was going through the album tracks on other dates.
I think you might be right, because when I heard the Pixies sang in Spanish at the beginning of the tune, I presumed it was Isla De Encanta, I didn't check it with Peel at the end of the song. However, I didn't realise the Pixies also had another Spanish title called Vamos. I think the Vamos verdict seems to have the stronger evidence.