1. Put a pan of salted water on to boil.
2. Cut both types of potatoes into 3cm cubes but keep them separate.
3. Add the normal potatoes to the boiling water and cook for 10 minutes, before adding the sweet potatoes and cooking for a further 15.
4. Once the potatoes are tender, drain and allow them to stand in a colander to steam for 5 minutes.
5. Using a potato ricer, rice the potatoes back into their still-warm pan. Alternatively, if you don't have a potato ricer, use a potato masher. Season with around 1/2 teaspoon of salt, then add the butter, cheddar and three-quarters of the stilton and stir well. Adjust the seasoning to taste and serve with the remaining stilton crumbled over the top.
500g potatoes, peeled
300g sweet potatoes, peeled
Salt and black pepper to taste