Just a suggestion (not sure if it will work) but maybe for Blyth to fully allocate deckers to the expresses Blyth could do:
1497 Blyth to Belmont with either a DB250 or one of the Ex Jesmond B7s heading the other direction
1493/1501-05 Blyth to Belmont with 7416/19-23/27 going the other direction. This would allow the Expresses to be fully allocated B7s and VDL Deckers.