Bramley Lemon Curd

As part of my plan to get through the cooking apple mountain I decided to make some Bramley Lemon Curd from the River Cottage Handbook on Preserves. It is a fairly straight forward recipe which of course starts with peeling, coring and chopping apples. I've been through a good few kilos of apples this summer so far and so this was what I thought was going to be he worst bit. 

So you cook the chopped apples with 100ml water and the grated rind of two lemons. 

Then you put the sugar, butter, and some lemon juice into a bowl above hot water with the apple puree and wait until the butter is just melted.

At this point you check the temperature carefully and add 200ml of beaten egg. I was very careful and added the eggs as I was supposed to.

It's supposed to take about 10 minutes to get up to temperature and thicken. I waited about 20 minutes and it was getting nowhere near so I put the mixture into a saucepan and heated it up some more, got it up to temperature and it still didn't thicken. I frantically googled lemon curd recipes and came up with nothing. I added some liquid pectin, nothing and so resorted to some cornflour which did work. 

I have to say despite the difficulties with the recipe this tastes amazing. It is like eating apples and lemon custard. Any tips for thickening lemon curd would be gratefully received.