Spiced mackerel with horseradish potatoes

INGREDIENTS

• 500g/1lb 2oz new potatoes
• 2 tbsp olive oil
• sea salt and freshly ground black pepper
• 2 shallots
• 2 tbsp soured cream
• 2 tbsp creamed horseradish
• 2 tbsp finely chopped fresh chives
• olive oil
• leaves, to serve watercress
• 8 mackerel
• 100g/4oz butter
• 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