For the crab and salmon roll, make a salmon mousse by blending half the salmon with a pinch of sea salt in a food processor. Add the double cream and process until smooth. Fold the crab meat into the mousse then mix in the herbs and citrus zests. Weigh into 4 portions (about 100g/3½oz each) and mould into sausage shapes.
Cut the pastry into four 17x15cm/6½x6in rectangles. Place the sausages on the pastry lengthways. Brush the edges of the pasty with egg wash. Fold over the pastry to encase the filling and crimp the edges with a fork to seal. Trim off any excess pastry and brush with two coats of egg wash. Store in the fridge until ready to cook (they can be made a day ahead).
Preheat the oven to 200C/180C Fan/Gas 6. Line a baking tray with baking paper.
Bake the pastries for 11 minutes, or until golden-brown and puffed up.
For the brown crab ketchup, squeeze out any excess water from the gelatine then add to a pan with the lemon juice. Dissolve over a very low heat then transfer to a blender. Add the brown crab, Worcestershire sauce, mustard, anchovy essence, ketchup and egg yolk then process until smooth. Add the breadcrumbs and continue to blend until smooth again. With the motor running slowly add the oils in a thin stead stream. Stop the machine and scrape the sides every so often. Season with salt and pepper. Pass through a fine sieve into a small jug or bowl.
Serve the warm rolls on a serving board with the ketchup alongside.