(23 Jul 2017, 3:00 pm)peter wrote I did think it had increased to 13 at some point, but then I looked through the timetable and only came out with 12! Nevermind! In that case 1512 would form part of the allocation and so my point still stands that perhaps a 13th bus will transfer from Ashington to act as a spare!

It was mentioned on here a while back that it was a PVR of 13, 12 deckers and one saloon. 

Back in 2014 when the 57 regs were being refurbished, there was an additional one refurbished as a spare, which gained X21/X22 branding, after the severe unreliability of the vehicles back then 7518 was refurbished, so its probable that another bus will transfer like you say.