I was going to write a whole lot more but i've just realised jac already said it....
yes, custard is a dessert. a very nice one. my kids love it. I add sugar to mine, otherwise its just not custard. I guess if you make it yourself you can control the sugar content, though from memory, the Heinz one doesnt contain much anyway. I think mine tastes better because I add more sugar.....

In terms of what its used for...well, its a dessert, so i guess its not strictly necessary. But l have no problem feeding it to my kids. They have it after their very healthy and nutritious main meal and it hasnt stopped them eating a wide variety of foods. Just because something contains sugar it doesnt mean its bad for a baby. I must say heinz baked beans are a godsend for me. If i'm disorganised my lot tend to get that for their lunch. You can get a no added salt variety....though the kids probably like it because they add sugar.
I had the annabel karmel book too. I didnt use it a huge amount. Some of her desserts were quite good. though they did contain *gasp* sugar...
You can get the Jalna yoghurt here Jac. We love the vanilla one in this house!