Thanks a lot everyone for the suggestions.
I hoped maybe there might be a magic solution for dissolving the metals into a non-toxic de-ionized state so that it could be just returned to the ground somewhere, or perhaps used sparingly as a fertiliser. We were having a chat last night here about how unthoughtful it is that we create things like metals, plastics and other chemicals without having any real responsible way of decomposing it.
Michael: the pigs idea is actually great. We do have wild boars roaming here digging up in the area which did already reveal some things - having some extra snouts to help dig up would help a lot. We think there might be a few tonnes of crap hiding under the ground.
Dale: I recall watching something by Paul Stamets for clearing up an old junk site with fungi. I will see what I can find - thanks for the tips about dangerous chemicals and junk yard folks.
Miles: The old farmers thing is exactly what has happened here, but it is from poor Soviet times in Estonia. I don't think there is much craigslist here

There is a scrap yard not too far from here though.
David: I was wondering about the recycling. Even if just doing some sort of DIY recycling of the raw material ourselves. For example, I know you can make an electrolyte solution (water and bicarb soda), put some rusty metal in the solution connected to a small 12v charge, with the other end connected to a metal attractor and the rust will simply dissolve off the metal and onto the rod, leaving you with a nice rust free piece. I thought if something that simple can be done to remove rust, surely there must be a simple way to naturally reduce all the metal (and rust) into something usable. I'm guessing you need a smelt for that.
Otherwise, the old farming tools in the soil are so old that they are barely recognisable.
Thanks for all the ideas everyone.