Sift in mixed spice and plain flour until well incorporated.
In a seperate clean bowl, beat egg whites until fluffy, then gradually add in the sugar and whisk until very stiff.
Add a big scoop of egg white mixture into the pumpkin mixture and mix well, gently fold in the remaining egg white mixture into the pumpkin mixture without deflating the mixture.
Mix in the pumpkin seeds.
Carefully pour into the mixture into the tin, smooth the top and bake for about 20-25 mins.
Remove the tin from the oven and invert the tin. Leave to cool and gently run a knife around the sides to remove from the baking tin.