I saw a thread on uk campsite which I thought you might be interested in. It shows pics of using the cobb and a storm kettle. The poster said they used the 'lighting up period' to boil the water. We are going to give this a go next time we camp and also try heating the water after we have used a (cheap and imitation) cobblestone to see if that creates enough heat. Just thought it might be of interest to you, certainly makes sense to use the energy if you can!