Why does boiling cream have such a drastic effect on my salted caramel? We have a recipe for a salted caramel spread that is basically a big pan of sugar, glucose, milk and butter, cooked until it becomes a thick paste, then mixed with double cream to achieve a smooth consistency and a glossy finish. Our only problem was that we kept having big variations between batches -- some ended up as thick, sticky spreads (like we intended), others were runny and sauce-like.

After some experiments we discovered that the final stage of the process -- pouring the double cream in -- was responsible. Pour the cream cold and the caramel would come out runny; heat the cream before pouring and the final result would be thick and rich.

Although we feel we're in control of the process now, I was wondering if anyone could explain to us why does heating or not heating the cream have such a big impact on the thickness of our spread.
You don't tell us neither the ratio of double cream to caramel, nor the time you heat the completed mix, so this is just a guess. But it sounds logical that your problem is evaporation. 

Double cream consists mainly of fat and water. I don't remember the exact percentages, but more than half of it is water. So, if you heat cream, part of it evaporates during the heating, and it continues evaporating after being added to the pan with bubbling caramel. But if you use cold cream, its water hasn't evaporated during a heating phase, and if you add a large amount of cream to a hot caramel mixture, the whole mixture cools considerably. If you don't heat it and let it simmer for long enough afterwards, the water content of the cream continues thinning the spread. You end up with a runny sauce instead of a sticky spread. 

Instead of pre-boiling, you should be able to just simmer for longer time after adding the cream, if you find it more convenient. But you'll have to try it a few times until you have found the cooking time which gives you optimal consistency.