Depending on the cost of Euro 6 conversion, would permanantly swapping 6x E400MMC from Ashington with 6x B9TL not be a solution?

B9TLs could then be allocated on the X14/X20 only. And with no evening & Sunday service (except the Sunday X14/X15 board which could be worked using E400MMCs), would at least conserve the B9TLs.

Don't know if these have USB charging? But Arriva being Arriva and liking to put anything on anything, could even put some on the X21/X22 which they'd cope with fine if they couldn't hack the X14/X20.