That tends to be their allocated depot for heavy maintenance & exams. When a full overhaul is needed, then works such as Loughborough, Holbeck, Doncaster, Crewe etc. come into play.
Northerns depot allocation policy seems to have changed.
Often the home depot was responsible for all units of a particular class even though it may be out at another depot who did just day to day maintenance.
Examples where Newton Heathes 150s and 156s and Neville hills 158s which popped up allover.
Recently things seem to have changed with some 158s allocated to Heaton and some 150s reported at Neville hill.
The new 331/1s where reported as allocated to Allerton even though they were for use at Leeds but they now seem to be at NL for at least day to day maintenance. All a bit of a puzzle.
K
P.s. anyone confirm the 150 156 158 units recently rumoured reallocated to Heaton and Neville Hill.