Melt the chocolate in a heatproof bowl over a pan of simmering water (be careful that the bowl doesn’t touch the water) or in a bowl in the microwave in short blasts, stirring regularly. Leave to cool for 5 minutes, then stir through the custard.

Meanwhile, put the cream in another large bowl with the orange zest and icing sugar. Whip, using electric beaters, until you get soft peaks.

Start building the trifle in a large trifle dish or glass bowl. Lay the Swiss roll slices across the base and up the sides of the dish. Drizzle over the juice from 1 tin of mandarins to soak into the sponge. Spread the marmalade over the sponge with the back of a spoon and scatter over the drained mandarin segments from the tin. Spoon over all of the chocolate custard.

Drain the other tin of mandarins, discarding the juice, and layer the fruit on top, reserving six or so segments for decoration. Then layer on half of the chocolate orange segments. Spoon over the whipped cream and decorate with the reserved mandarin segments and all but one of the remaining chocolate orange segments. Use the remaining chocolate orange segment to grate over the top of the trifle. Chill for at least 4 hours or until ready to serve. 

More ideas:
Chocolate Orange fondant
Gingerbread trifle