Place the cake tin ring (but not the base) onto a serving plate. Place a strip of acetate plastic (or plastic wrap) around the inside of the springform tin.
Slice the sponge in half horizontally, creating two slim discs of cake. The cut must be as level as possible as it will be visible in the finished cake. Place one layer of sponge cake in the bottom of the cake tin (cut side up). Then liberally brush the sponge with half the syrup.
With the back of a spoon, gently squash the edges of the cake down so that they are pushed directly against the sides of the tin, creating the defined edges necessary for the Fraisier cake.
Rinse, hull and halve about 12-15 strawberries, try and make sure they are all the same height.
Place the cut sides of the strawberries on top of the sponge base, against the acetate on the inside of the tin. The strawberry halves should be sitting snugly beside each other.
Take the piping bag of chilled crème pâtissière out of the fridge and snip off the end of the bag.
Pipe a swirl covering the exposed sponge completely in the bottom of the tin.
Then pipe between each of the strawberries so the gaps are filled right to the top with the crème pâtissière.
Use an offset spatula to gently level the creme and push it into the crevices around the strawberries.
Set about 3-5 strawberries to one side for decoration, then hull and chop the rest of them and place evenly on top of the crème.
Pipe another swirl of crème pâtissière on top of the cut strawberries to cover the whole surface, then smooth with a palette knife.
Place the other disc of sponge on top of this, with the cut side uppermost, so it has a completely flat top. Brush with the remaining syrup.
Gently press the top down, so that the cake and filling push against the acetate.
Lay the chilled marzipan circle on top of the cake. Then put the whole thing back in the fridge to set for at least 2-3 hours.
Meanwhile, make some decorations of your choice with melted dark or white chocolate or make some marzipan cutouts to lay on top (I added a couple of extra drops of colouring to some of the marzipan scraps to make a darker shade of pink, made some star cutouts, and adhered them to the cake with some of the lemon syrup).
When ready to serve, remove the cake from fridge. Very carefully release the spring tin/loose bottom and remove the cake from the tin and from the acetate or cling film. The custard will probably need a bit of a touch-up as it may adhere partly to the acetate – simply use a warm offset spatula to smooth it out.
Decorate the top with reserved strawberries and chocolate or marzipan decorations. Serve immediately. NB: The cake can be stored in the fridge for a further day.