According to bustimes, Northstar has gained another Mellor Strata which used to be with Arriva.
So I guess the 558/568/515 will be ran by these then as I'm guessing due to the newest being fleet numbered "14" and the one used on the 520 is "11", that 2 more will be coming.
If they don't come soon (as the contract is very soon) then the 2 small E200s could also be used tbf...
RE: Northstar