813's with Arriva (London Overground), not too sure what the story with that one is but it shouldn't be withdrawn yet since the 710s are quite a way off from being introduced?
I'm pretty sure I have seen 828 several times as of late. 845 I think is in the depot for some sort of work as it apparently has been faulty (not too sure what the specifics are) quite a bit lately otherwise I have seen it sitting in the Alderbrook Carriage Sidings. It should return to service soon I think.
As for 827, that one is one I just tend to rarely see, even when we had a full compliment of 61 315s. I will be on a look out for all 4 of these units and I'll report it if I spot any.