Sorry Simon it doesn't have that feel for me. Two thoughts on rubble: one I have used sand mixed with Spackle with pieces of plastic spure and bits of foam pressed in to simulate concrete rubble, two place a plastic or mdf disc over the base cut the disc in half make it a jagged cut then glue the halves onto the base be sure to leave a "crack" a few millimeters wide paint it up like macadam and glue static grass growing out of the crack.
I hope this helps.
Rob