(14 Feb 2015, 8:51 pm)mb134 wrote May I offer a possibility, for the state of the X21/22 anyway?
Ashington allocated 7518 to the X14 early in the morning, two of the Sapphire E400's the failed, so 7491 and a MAX E400 replaced them?
Doesn't explain, of course, why 7518 wasn't switched at some point in the day.  

The two standard E400's could have been at the Alnwick outstation if two MAX ones were at Ashington getting attention, one of which may be the one allocated to the X21/22?

Problem is sometimes it can be difficult to switch buses over, it was the same on the 7 on Tuesday when Darlington depot had 1427 on the 7 while a Sapphire Pulsar was on the X1 and the times when they arrived in Darlington were at different times and a non Sapphire bus is still better than no bus at all.

I know non Sapphire/MAX buses on Sapphire/MAX services are not ideal but it's preferable to no bus.

It could be worse, at least most of the wrong types of buses have at least been the same sized buses, meanwhile in Darlington we are having Solos appearing on routes that should be allocated full size single deckers.