(Yesterday, 8:18 pm)Superman wrote Not an excuse for this, but instead an educated observation I can hopefully share.
Arriva have almost every location with an outdoor depot layout. Only Durham is partially indoor, but still half of the fleet is outside all night. This time of year, temps overnight get down to freezing, regularly under freezing.
Running a bus through the wash at 10pm/l or midnight and immediately parking it up will mean the bus simply freezes. It doesn't dry. This causes big issues with frozen axles, air pipes and such at 6am just a few hours later. Almost certainly, they will have had experience of this in the past and decide not to wash them on some evenings, possibly a days few back to back. Prioritised service over cleanliness, is likely the thought.
Again, not an excuse for what might not be driven by this but sometimes we need to think about why this might actually be happening and it could be something as simple as that.
To be fair, the roads in County Durham are absolutely mankey aswell. I'm not sure what type of grit they use but it seems to be full of sand. Noticed it's been like that for awhile whereas the North of Tyne council areas are more salt.
You could wash a bus down there and 20 minutes later it'll be dirty again anyway.