Beat eggs on high speed for five minutes. Gradually beat in sugar and stir in the pumpkin and lemon juice. In another bowl mix together the dry ingredients and fold into cake mixture.
Line a greased Swiss roll tin (40x25x2.5cm) with baking parchment then grease the parchment. Spread the cake mixture evenly in the tin and sprinkle with the chopped walnuts.
Bake in preheated oven for 12 to 15 minutes.
Turn out on to a tea towel dusted with icing sugar. Remove the parchment and starting at the narrow side of the cake, roll up the cloth and cake together and leave to cool.
Filling:
Whip all ingredients for the filling together, until well combined.
Unroll sponge when cool and spread with the filling, then back roll up. Chill finished roll before serving. This freezes very well.