Put the sweet potatoes in a large pot, cover with cold water and simmer over medium-high heat until tender.
While the potatoes are cooking, combine the sugar, water and kumquats and boil until syrupy.
When the sweet potatoes are tender, drain them well in a colander and return them to the pot. Season with salt and nutmeg and mash to your liking. Add butter and cream, again to your liking (I like mine without).
Place the mashed sweet potatoes in a serving dish and scatter the candied kumquats on top. Any leftover syrup can join the duck glaze or be used in making chutney. This can be made the day before and reheated in the oven, uncovered, so a little crust forms.