Fry the pancetta in batches until it begins to crisp. Drain and set aside.
Using a Parisienne scoop, make balls of potato (you want 20 balls in all). Put these in a pan and cover with water. Add the saffron and a good pinch of salt and braise over a low heat until soft.
In a heavy-bottomed pan, warm some oil and fry the fennel wedges on a medium heat until they start to soften and caramelise (around 10 minutes). Add the white wine and reduce by half.
Add in the pancetta and fish stock and reduce by a third.
Meanwhile, preheat the oven to 180C/350F/gas mark 4.
Place the cod on a sheet of tin foil and brush it all over with the melted butter, then add a squeeze of lemon, a pinch of salt, and a small handful of capers. Wrap up in the foil to create a parcel, and cook for 12-15 minutes, depending on the thickness of the cod.
Add the cucumber and celery to the fennel pan. Simmer for 3 minutes until the vegetables begin to soften, but still have bite.
To serve, place the fennel, cucumber and celery mix on the plate in a neat pile, and lay the cod on top. Pour the broth around the outside, and garnish with saffron potatoes and fresh radish.