Posted: 11/02/2017 at 14:25
if that black stuff is plastic sheet I'd get as much of that off as you can. Lot's of things grow in rubble, (trust me, my garden is built on rubble), brick rubble is OK but plastic sheet isn't good.
For wildflife, Viper's Bugloss, lovely blue biennial but you can get a small annual version that would get you flowers this year. I'd start with a selection of cheap and cherrful annuals and see what does well.
If it's a solid base with cracks, seeds are the best way forward