They say nettles grow round places used by us humans, even a long time ago. A lot of wildlife uses nettles to breed. So maybe wait till not actively growing. Find out at the GC what can be used to kill them in the ground at the root. Sounds like you might need a tankerful of whatever kills them.....good luck!

I think it would be better to get well dressed up in boots, long thick trousers, thick shirt and gloves and get in there. Maybe with a face mask if they're really tall. I have a long-handled scythe that is just the job for mowing things like nettles about 3 inches above ground level. Then you need to get as much of the root systems out as possible. The nettle top growth could be raked up to make good compost and the roots could be dried and burnt. You'd end up with a usable plot quicker than if you used a whole lot of weedkiller. You'd still need to clear the dead growth off it anyway, and the roots would stage a comeback, no matter what weedkiller you used.

Whether you clear the ground by the "chop off and dig out" method, or by the "spray, chop and dig" method, you'll need to watch for regrowth and be ready to dig out surviving roots for several months. It's a big area and whatever you do it's going to take work, but they reckon that nettles growing is a sign of fertile soil, so it should be worth it.

Send some my way if you can. :-)