For the batter mix all the flour and dried ingredients together in a bowl. Make a well in the centre and whisk in the beer and spring water. Leave to rest. In a mixer whiz the egg yolks until creamy. Slowly add the oil, blending all the time, until the mixture is emulsified. Remove from the mixer and fold in the olives, anchovies, capers and gherkins. Mix well so you have a textured mayonnaise, season with salt and pepper.
For the puree melt the butter in a pan, add the peas and cook until softened. Remove and blend until smooth. Finish with the freshly chopped mint and lemon juice. Season to taste.
Cut the potatoes into chip size pieces. Blanch in oil at 100c until soft. Remove and lay on greaseproof paper until cool. When ready to serve, flash fry the chips at 180c until crisp.
Dip the fish pieces into the beer batter and deep fry until crisp and golden brown.
Serve the fish with the chips, pea puree, olive mayonnaise and garnish with chervil.