A few corrections and debunking fan-myths:
Spearhead from Space: Both the negatives and
positives were retained.
The Abominable Snowmen 2 and Invasion of the Dinosaurs 1: Both were recovered by Roger Stevens (along with five other episodes) and were handed over to Ian Levine, but Ian chose to retain Dinosaurs 1 as bargaining material for possible later finds. As none came, he returned it the following year.
The Ice Warriors 1: The 'incorrect label' thing is actually only a single hand written note in one corner saying it was part two; the rest of the details on the label refer to part one. This was likely added in error when the films were found in 1988.
The Celestial Toymaker 4: It was leftover from ABC's broadcasts, NOT from RTS in Singapore. Jon Preddle (who runs the BroaDWcast website) informed me this will be corrected in the new version of Wiped! next month.
The Wheel in Space 3 was found by David Stead, who would also recover the unedited and PAL 2" copy of Death to the Daleks 1 in 1991 (which had been abandoned on a loading dock!). Up until then only a 2" NTSC and 1" edited PAL version existed.