Dig it up. Use a fork, loosen it from the edges. Each large clump is made up of lots of smaller ones. When you can see one of these pull it up. The first one is usually the hardest, after that it's just a case of divide and conquer!
Watch out for seedlings. They prefer damp ground but will grow almost anywhere.
Ah yes now that stuff I had a lot of, if your mower will, mow it. it doesn't survive more than a couple of months when mown. It also dies pretty fast (6months) under black plastic if it's an area you cannot mow.
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https://www.naturespot.org.uk/species/soft-rush
If so, that's what it is.
The best way of getting rid of it is to improve the drainage.
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.