Basically I want to leave my Dark Brotherhood armour in the DB sanctuary for a short while. Will it still be there when I go back?
I understand that if you leave stuff in random chests there is no guarantee it will stay there, but are faction "hideouts" the same?

I'm assuming you're talking about the Falkreath sanctuary.
You can safely store things there, as long as you don't advance the DB questline. Something happens to that sanctuary later and if you have any stuff there it will be lost

Excellent, thanks very much.