To make your bases melt the butter and marshmallows in a large saucepan. Once melted, gently stir in the Rice Krispies. Pour the mixture onto a lined 30cm x 20cm tray and press down firmly to compact the Rice Krispies and make flat. Leave to set for 1-2 hours.
Once set, cut out 12 small circles with a small round cutter using a small sharp knife to cut all the way through to the bottom if needed. Use the rest of the mix to make 10 rectangular pieces.
Cut your cola belts to size, measuring them around the cutter. Stick them with a small smudge of melted chocolate. Repeat on all 10 and leave to set. Mix the millions and jam together and put small heaps on 6 of the circles to look like caviar. On the remaining 6 pieces, get a chopstick and make 4 indents in the centre of the rice. Cut your pencil sweets into small pieces and poke them into the indents alternating in colour.
To decorate the rectangles, split your fondant icing in half and colour half orange and half a purple/red colour so they look like salmon and tuna. Roll each out to 5mm thick. Use a knife to cut out shapes that will fit on top of the rectangles and use the back of a knife to make fine imprints to make the pieces more flesh-like. On the orange 'salmon' pieces, paint the imprints with the white colouring powder. Place all the bits of decorated icing on top of the rectangular bases, moulding them to look as natural as possible.
Use the green food colouring powder to brush the cola belts and create a seaweed effect.
Finally to make your 'soy sauce' heat the black treacle and 50ml water in a pan until combined. Brush over the 'tuna' and 'salmon' sushi pieces and put the rest in a bowl as a dipping sauce.