An update - I have potted out the fern. I lined the pot with plastic and made some holes, and used a mix of soil and compost with some tiny stones mixed in.
I'm quite pleased with it. You see more of the side of the leaves than the prettier top-down view of them, but I suppose that's unavoidable with a plant at that height.
When I watered it in, there was a trail of dirty water with soil that marked the pot and wall. Had to clean that up but hopefully the soil will bed in after a while and that will stop happening.
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