While the basic process for making a custard or crème anglaise generally stays the same, the ratio of sugar, milk or cream and egg yolk will vary across different recipes, as it is largely down to personal preference.
Using only double cream, as both Marcus Wareing and Chris Horridge seem to prefer, makes the consistency of the custard thicker, whereas a mix of whole milk and cream results in a thinner consistency more suited to pouring over a dessert. The amount of egg yolks used will also have an impact (the more you use, the richer the flavour will be), while the amount of sugar used should depend largely on how sweet you prefer your custard to be. Vanilla is a classic flavouring for custard, and recipes generally call for pods to be used. If you can’t get hold of pods, however, use vanilla essence instead.