This isn't factual and is opinions but imo they should, even known it's a bit long winded:

4x Enviro 400 MMC, Ashington to Redcar

1479/1512/1513, Ashington to Durham
7523/1579/1580, Durham to Ashington

1584/1585/1586/1587/1588/1589 - Darlington to Ashington
1537/1540/1541/1542 - Ashington to Darlington
1538/1539 - Reserve to Darlington

1404/1405 - Stockton to Ashington (Spares)

Enviro 300's split between Stockton and Darlington as spares
Redcar B7's withdrawn

It's the best way to split the damage across easy depot, and get some consistancy of where things are ie. Sapphire Pulsar's all together at Durham, all the 14 plate Enviro 400's together etc.