An interesting post on the 350/2 thread shows a recent'ish aerial view of Long Marston.
Rail Magazine is correct regarding 350/2 seating capacity being lower than 323s. 350/2s have 270 seats (246 Standard Class, 24 First Class) 323s have 277 seats. Most of the Northern Network is set up as a 6 car Railway, so without SDO/ASDO the 350/2s can’t lengthen any services with Northern.
www.railforums.co.uk
I do not know whether this covers all of the storage roads at Long Marston, but only a single 458/5 unit is visible. So either the other four 458/5s (prior to today) are elsewhere within the complex, or they have gone elsewhere full stop.