They are now hanging off a hot spot downloading there 1.8Gb each extremely slowly. Well it went from 18 hours on ours to 48hous on the hotspot.
As
Old Speckled Aircrew said, use iTunes.
iTunes simply does a backup/restore and images the new iOS 8 into the phone.
I looked at the (notalotof) spare space on my two iThings, looked at the added space it occupies, and decided I don't need it anyway.
You took the wrong approach.
It overwrites the old operating system, therefore the space question is irrelevant !
The space question only applies if you're doing a live upgrade instead of via iTunes.