Heat the Madeira (wine from Madeira) in a small saucepan and cook until it has reduced by half.
In a separate pan, heat the lemon juice and water. Melt half of the cold margarine into the lemon juice until it reaches a creamy consistency and season with sea salt and ground white pepper. Set aside, keeping it warm.
Scatter the flour over a plate or clean surface and put the mackerel fillets in the flour. Pat off any excess flour.
Heat the olive oil in a clean pan and fry the mackerel fillets for a couple of minutes on each side. Remove them from the pan.
Melt the remainder of the margarine in the pan used to fry the mackerel and add the capers.
Leave for a minute to heat through.
To serve, spoon the buttery lemon juice and capers over the fish, trickle the reduced Madeira over, and sprinkle the chopped parsley on top.