Halve the strawberries if they are large. Place in the blender with the yoghurt and icing sugar
and pulse until smooth.
Push through a wide-meshed sieve to remove the strawberry
seeds.
Rinse out the blender, add the yolks and whizz.
In a small pan, dissolve the caster sugar in the water over a
low heat and bring to the boil.
After about seven minutes, when reduced by two-thirds, the
bubbles in the syrup will shine and flatten. This is called boiling
to the thread stage.
Turn the blender immediately on to Speed 1 and add the hot
syrup gradually to the yolks.
Tip in the yogurt and the strawberries, and blend until smooth
and dusky pink. Scrape into a metal bowl, and leave to cool in the
fridge.
When the mixture is cold, churn in an ice cream maker until
frozen (it may seem slushy when done). Transfer to an air-tight tub
and store in the freezer until completely firm.
Allow to soften in the fridge for about 20 minutes or so before
serving, or give one or two rotations in the microwave.