We’ve got loads of building rubble from some work on the house. Down the side of the garden is a foot long trench that I intend to fill using said rubble. Anything I need to be aware of? Will rainwater just wash any topsoil into the voids? Do I need to put loads of other stuff down too?

Should be fine. I did the same years ago to help drain our garden just put the bigger bits in first and the smaller stuff on top will prevent the soil washing through.

I’d consider putting a separator geotextile between the rubble and whatever’s underneath the rubble. It depends what is underneath the rubble.