When fresh berries are in season, it's hard to keep them around long enough to make anything like a pie with them. They keep disappearing, eaten a handful at a time by everyone passing by the refrigerator. This is a nice, quick way to enjoy the berries in a dessert before they all disappear.
If you don't keep caster (extra fine) sugar in the house, you can substitute regular sugar. You might want to try to dissolve it a little first with the lemon extract before you add the egg yolks. Then, you may have to whisk it a little longer before you put it in the hot water bath. It's not a big problem - you may just get a bit of sugar 'crunch' if the sugar doesn't dissolve all the way. It's all still very tasty!