I once visited a site in North Wales where land remediation was being carried out on an old slate quarry tip. They tries a range of soil conditioners such as green waste compost, paper mulch, slate powder and manure. The treatments all had varying degrees of effectiveness, but not the paper mulch or slate powder. I have friends up here with similar problems that just tipped anything they could get their hands on, to an old slate tip. They had varing success, but I think that importing soil is a good way to go.