Spiced mackerel with horseradish potatoes

Name: Spiced mackerel with horseradish potatoes
Ingredients: 500g/1lb 2oz new potatoes, cooked in their skins; 2 tbsp olive oil; sea salt and freshly ground black pepper; 2 shallots, finely sliced; 2 tbsp soured cream; 2 tbsp creamed horseradish; 2 tbsp finely chopped fresh chives; olive oil; watercress leaves, to serve; 8 mackerel fillets; 100g/4oz butter, softened to room temperature; 1 tsp cayenne pepper; 2 tsp sweet smoked paprika; 1 tsp ground coriander; 1 tbsp golden caster sugar; ½ tsp English mustard; few drops Tabasco; 2 tsp red wine vinegar