The newer stock is requied in Fife for the school contracts which have a 15 year age limit.
All the Scanias from London are destined for Aberhill.
Did a quick 'back of a fag packet' count (I don't claim the numbers are 100% up to date), so: -
Rennies have: - School pvr of 24 tridents, with actual numbers as 2 '02 reg, 9 '53 reg, 9 '04 reg.
Dunfermline have: School pvr of 6 Tridents, with actual numbers as 2 '53 reg, 2 '04 reg, 2, 55 reg.
Aberhill have School pvr of 17 tridents, with actual numbers as 9 '04 reg, , 3 '54 reg, 5 '55 reg.
Glenrothes have School pvr of 1 Trident, actual number 1 '04.
( I an excluding the scheduled School pvr for E400s and except that there will be some operational overlap).
All the '02, '53 reg are already over 15 years old and the '04s all reach that age this year, with the '54s later in the year.
So that's a total of @ 35 Tridents + 4 '54 reg tipping over into 2020.
At the moment there's planned to be 15? ex London Scania to replace at Aberhill, so presumably at least another 20 newer deckers needed to keep the fleet inside 15 years.