I've given up on the X15 and X18 allocations now. Having a bus on the route its actually branded for is rarer than it being somewhere else. Or at least it feels like it.
On Thursday 7511 was on the X18 and 7520 was on the X15. God help them come 24th May when they're adding more bus(es) to the X18 for the summer, especially if they're going to try and manage with the buses they already have.
RE: Arriva North East: Rare & Odd Workings - February 2015