My butterscotch pie filling curdled?

I was making a butterscotch pie for the weekend, by following a recipe from the net. The ingredient list was

• 1 cup dark brown sugar
• 1/4 cup cornstarch
• 1/4 teaspoon salt
• 4 cups half-and-half cream -- (used UK double cream)
• 5 egg yolks , seperated slightly beaten save whites for Meringue
• 1/4 cup butter , sliced up
• 2 teaspoons vanilla extract

I followed the instructions (I think correctly).

1. In saucepan combine brown sugar, cornstarch, and salt. Whisk in half & half.
2. Whisk constantly over medium heat till mix is thick and bubbly.
3. Cook an additional 2 minutes. Remove from heat.
4. Gradually whisk about 1 cup of the hot mix into the egg yolks, whisking all the time.
5. Add this back into the rest of what is in the pan.
6. Bring to a gentle boil. Reduce heat and cook and whisk for an additional 2 minutes.
7. Remove from heat and whisk in butter and vanilla till well mixed.
8. Let cool a bit, whisk and pour into pie shell and set aside.

Just after the last remove from heat and just before whisking in the butter, I needed a call of nature. When I got back the mixture had separated into what looked like curdled milk and an oily fat like substance. I tried just whisking the lot, but it refused to recombine, so I poured off the oil. The remaining substance (with a little oil) whisked fine when reheated slightly, so I added the butter and vanilla and carried on.

The pie came out tasting fine. But after the pies had been topped and cooled, there was a slight layer of oil onto of the set butterscotch, but beneath the meringue. Pouring the oil off the pies gently, got rid of that problem.

This is the first time that I have had such a monumental departure from a recipe I have been following (probably luck so far). But can anyone see anything wrong with either the recipe / instructions or suggest what I have did wrong.