(17 Jan 2017, 7:55 pm)L469 YVK wrote If they replace them, I think the most logical solution would be to replace 7534-40 at Darlington / Belmont with new Enviro 400 MMC's then  use 7534-40 plus 7518 to replace 7501-09, therefore keeping all  vehicles the same type.

2x best performers of 7501-09 get kept back as spares with the rest being dispersed nationally to other Arriva divisions with double deck Sapphire services.

That would leave 2 vehicles that aren't famed for their reliability being relied upon as spares? 

The whole point of getting 7501-9 away from the X21/22 would be to improve reliability. To maximise the probability of this, you'd give it a new allocation of vehicles, in addition to 7522-8.