I've just made the worst bundt cake. The crust was like a rock surrounding the tender cake, what makes a crust so hard on cakes and muffins?

Are you using a fan assisted oven? That can seriously mess with baking - you may want to reduce the temp a bit or turn the fan off. Most recipes give the temperature for a conventional oven.

Also, even if your oven is not fan as sites, it still probably won't be calibrated exactly the same as the recipe author's oven. Sounds like oven might be too hot.

Sometimes it has to do with the type of pan you use. For example, the dark colored non-stick pans require a shorter baking time; otherwise you end up over-cooking the outside edge.