First of all whip the double cream until it reaches the 'floppy' stage but isn't too thick, then pop it into the fridge to chill. At the same time, put the polythene box into the freezer to chill as well.
Now make the custard by first pouring the single cream into a saucepan and then heating it up to boiling point. While that's happening beat together the egg yolks, vanilla extract, custard powder and sugar until absolutely smooth. Next pour the hot cream on to this mixture, whisking as you pour. Now return the custard to the pan and continue to whisk it over a medium heat until it has thickened and come up to boiling point again. (Ignore any curdled appearance, which may come about if you don't keep stirring and have the heat too high. The custard powder will stabilise it provided you beat it off the heat: poured into a bowl it will become quite smooth again.)
Now place the bowl of custard in a bowl of cold water, and stir it now and then until absolutely cold. Then fold in the chilled whipped cream. Pour the whole lot into the chilled polythene box, cover and freeze for a couple of hours or until it is just beginning to set.
At the same time place a mixing bowl in the freezer to chill, then as soon as the mixture does begin to set, tip it into the chilled bowl and whisk (with an electric hand whisk if you have one) very thoroughly. Then return the ice cream to the polythene box, put it back in the freezer (covered) and leave until frozen (about 3 hours).
To make the praline, place the sugar and almonds together in a pan over a low heat, and leave them until all the crystals of sugar have completely dissolved to a liquid. Cook until the liquid has turned a rich brown colour. Then pour this mixture on to the baking sheet, spreading the almonds out in a single layer. Leave the mixture to cool and become brittle; then, using a palette knife, lift it off the baking tray on to a flat surface. Break it up with a few bashes from a rolling pin, then crush it fairly finely with the rolling pin.
Remove the ice cream to the main body of the fridge for about 15 minutes to soften slightly then stir the crushed praline thoroughly into the ice cream about half an hour before serving.
You can, if you like, make the praline at any time and store it wrapped in foil in the fridge.