Shame you missed the chance to sow in autumn, it’s a quick job. Even now (tomorrow) you could sow & simply rough rake, in Cornwall it’s mild enough, many surface-lying seeds will germinate.
Then mow in March. Regular mowing will weaken thistles, nettles etc.
Then yes say 14T compost in autumn, polythene over, in spring ’18 lift a corner of poly to se what is growing…. perhaps add more compost to a small area for intensive growing, before lifting the rest of the cover, be sure to kill perennial weeds.
