what the heck is gas mark 1/2?

If you're not just interested in the how but the why, let me point you at Cooking for Engineers. Some parts of the site are more satisfying than others; for example, the "ingredients directory" mostly just shows you what various vegetables look like, and seems unfinished. But there are also useful articles on things like the smoke point of different fats, how to decipher USDA beef grades, ingredient substitutions, how to make butter (with cool science about why it works!), and--especially useful for those of you who have expatriated, or are using a cookbook from a country that uses a different system than you're used to, a conversion chart for oven temperatures.