Line a 23cm springform cake tin with baking parchment and lightly butter the sides.
Tip half of the biscuits into a zip-seal bag or large mixing bowl and bash with a heavy implement (but don’t be stupid please, safety first) until the biscuits are crushed to fine crumbs. Pour into another bowl and repeat with the second half of biscuits.
In a saucepan, heat the butter and golden syrup until the butter has melted and the syrup has thinned. Stir into the biscuits until all the crumbs are evenly coated. Spoon into the cake tin and press down until firm and even. And maybe eat a spoonful.
Drain the mandarin segments and evenly coat the base with them, reserving nine or ten for decoration. Pop the cake tin in the fridge to set as you make the filling.
In a mixing bowl beat the mascarpone until smooth, then add the fromage frais and sugar and stir all together.
Finely grate the zest of the lemon and lime and add to the mascarpone mixture along with both fruits’ juice. Gently stir to combine.
Whip the cream with electric beaters until you have soft peaks – the cream should flop back on itself. Fold it gently into the zesty mascarpone. It will look irresistable so taste some... for quality control purposes.
Retrieve the base from the fridge and cover it with the soft creamy filling, spreading it evenly to the edges and level the surface. Cover the tin with a sheet of foil and return to the fridge to set for at least four hours or overnight.
Once set, slide a cutlery knife around the edge of the tin to loosen the cheesecake. Carefully remove it and set it on a plate. Now, you can decorate! Whip the remaining 100ml double cream until thick, then either pipe swirls or spoon blobs around the edge. Garnish with a reserved mandarin segment. Crush a couple of ginger biscuits and lightly scatter around the border, along with some freshly grated lemon or lime zest.