Thinly slice the salmon down on to the skin, allow once slice per person. Cut the mackerel into diamond shaped pieces. Divide the trout into large flakes. Skin and slice the eel. Slice the tuna and the hake thinly. Horseradish mayonnaise: Put the grated horseradish into a bowl with the vinegar, lemon juice, mustard, salt, pepper and sugar. Fold in the softly whipped cream but do not over mix or the sauce will curdle. It keeps for 2-3 days but cover it tightly so that it does not pick up other flavours in the refrigerator. To serve, choose four large plates. Put a blob of horseradish mayonnaise on each plate. Garnish with a lemon wedge and sprigs of watercress or rocket leaves. DISCLAIMER(c) Copyright 1996 - SelecTV Cable Limited. All rights reserved. Carlton Food Network http://www.cfn.co.uk/