Abneyhave some excess topsoil at the moment. They bought too much for their children's garden project. Just help yourself from the builders bags behind the hedge, next to the drinking fountain 100m from main gate (Stamford Hill).
I'm not sure of the position regarding free compost since LBH evicted ELCRP from the marhses to build a car park but I suspect there is still free compost to be had if you time your visit when the parks yard is opnen.
That should do me for the moment!