Dissolve jelly in very hot water according to packet directions then allow to cool. Put 4 tablespoons of cooled (but still runny) jelly in a 1 litre glass bowl. Top with 5 slices of Swiss roll. Swirl around continuously until set. Place remaining slices of Swiss roll onto set jelly.
Whisk cream until very thick and add remaining cooled jelly. Stir with a fork until well mixed. Spoon this jelly-cream mix onto Swiss roll layer and transfer to fridge until set - about 1 1/2 hours.