From the booklet in the TUF boxset:
Disappearing Act,’ formerly known as ‘White City,’ was recorded at the beginning of the Slane sessions before we had set up our multi-track recording equipment, hence it only existed as a stereo instrumental recording. We did multi-track tape recordings of a simplified version later on, but for whatever reason, it never inspired a compelling vocal melody idea; maybe because when we came to re-cut the track, we omitted the section before the chorus, which in ‘Disappearing Act’ turned out to be a crucial section. So after 25 years and a few weeks, while in France on a break between shows on the 360 tour, ‘White City’ was completed and became ‘Disappearing Act.