Reserve 12 raspberries for decoration and place the rest in a food processor with a good squeeze of lemon juice.
Blitz to a puree, then push through a sieve to remove the seeds. Add icing sugar to taste, leaving the fruit slightly on the tart side, then set aside.
Place the egg yolks and caster sugar in a large, heatproof bowl. Beat in the sherry and wine, using a balloon whisk.
Place over a pan of simmering water and heat gently, whisking continuously until the mixture is very light but holds its shape. When it is the consistency of semi-melted ice cream, take it off the heat and continue to whisk over a bowl of iced water until cool - this prevents it from splitting.
Fold the yoghurt into the mixture. Drizzle in the raspberry puree and gently swirl to create a ripple effect. Spoon into stemmed serving glasses, decorate with the reserved raspberries and serve with wafer thin biscuits, if you like.