Preheat oven 160C/320F/Gas 3.
Grease and line the base and sides of an 20cm/8in deep, round cake tin with baking parchment.
Measure all the cake ingredients into a large mixing bowl and beat well until thoroughly blended. Place half the mixture into the prepared tin and level the surface.
Take one third of the marzipan and roll into a circle the same size as the cake tin, place the circle on top of the cake mixture. Spoon the remaining mixture on top of the marzipan and level the surface.
Bake for about one and three-quarter to two hours or until golden brown and firm in the middle. If toward the end of the cooking time the cake is getting too brown, loosely cover with a piece of foil. Allow the cake to cool in the tin before turning onto a cooling rack.
When the cake is cool. Brush the top with a little warmed apricot jam. Roll out half the remaining marzipan to the size of the cake and sit it on the top. Crimp the edges of the marzipan and make a lattice pattern in the centre of the marzipan using a sharp knife. Make 11 even sized balls from the remaining marzipan and arrange around the edge.
Brush with beaten egg and glaze under a hot grill for about five minutes, turning the cake round so it browns evenly, so the marzipan is tinged brown all over. (You can also do this with a blow torch if preferred)
To crystallise the flowers, lightly whisk the egg white in a bowl then carefully brush over the flower petals.
Sprinkle over caster sugar so the sugar sticks to the egg white. Leave to harden in a warm place, such as a shelf above a radiator or in an airing cupboard, until dry and firm.
Carefully remove from the rack and arrange in the centre of the cake.