The B5TLs shouldn't be anywhere near the X21, all good putting down to external factors but if roughly 50% of the PVR is off the road regularly, then something needs to be asked.

Yes the E400MMC & StreetDecks aren't 'perfect' but they're proven machines. Arriva have happily used E400MMCs and E40D's (classics) out of Ashington either on the X21/X22 or up the Coast on the X15/X18 without major trouble although some are getting on now due to their age.

I've said this before but if there's future changes in Derwentside and a split of the X30/X31 (PVR 5x standalone) and X70/X71/X72 (PVR 7x standalone), it would make sense swapping 6343-51 with 6308-14, 34, 35. As long as 6336 could be released from Riverside so that there's a low bridge decker to cover for 6338-42 on the X30/X31, that would still give the same spare ratio as now at Consett with 2x B5TL (6334-35) and 1x E400MMC (6336) plus 6064 as a last resort.