Having spent the last few weeks clearing the whole area, and digging it over, this is how it now looks:
I had to remove lots of large roots which was pretty hard work, and an old pond which still had some water in, and turned out to be the home of five frogs and two newts. They have all been re-housed at the local wildlife reserve so hopefully they are very happy.
I dug the whole thing over as deep as I could with a spade. By the time I had got about two thirds of the way through I wished I had hired a rotovator, but by that point it wasn’t worth it so I just kept going.
The whole thing has been raked out flat and seeded with grass.