Apparently mushy peas can be made in the slow cooker. The marrowfat peas still need to be soaked then drained as instructed in this recipe. After this, they can be placed in a slow cooker, covered with boiling water and left to cook on low for around 5 hours. They will need to be stirred every so often and topped up with a little more water if the pot is getting too dry. Add the salt at the end of cooking to avoid the peas getting tough and stir for 1-2 minutes at the end of cooking to break them down and achieve a good mushy consistency. Having said this, I wouldn't recommend using the slow cooker method for anything less than 8 servings - that's double the quantity listed in this recipe.