The instructions say gas mark 5 for 30 mins can you cook at 3 longer?

It's best not to simply add 3 minutes to a cooking time without understanding why it's there in the first place.

Here's why:

* Gas marks are not exact: They're a rough guide, and ovens vary in their temperature.

* Cooking time depends on many factors: The size and shape of your food, the type of food, and the desired level of doneness all impact the cooking time.

Instead of just adding time, consider these options:

* Check the food: Use a thermometer to check the internal temperature of your food, especially for meat and poultry.

* Look for visual cues: Does the food look cooked? Is it browning as it should?

* Adjust the time gradually: Add a few minutes, check, and add more if needed.

If you're unsure, it's always better to err on the side of undercooking rather than overcooking.