Female muscovy duck I'd say. Beauty is in the eye of the beholder when it comes to the drake of the species.
Hi Vicki, I think this is a Muscovy Duck. They are native to central and south america. They have been used for domestic purposes for many years and there are some feral populations.
The red wattles at the base of the beak is very characteristic and can look quite ugly. It doesn't look so pronounced on your bird so it's possible it may be youngish.
EDIT post crossed with JohnB
Thank you both. Lovely to know. It’s been hanging around a campsite in SW Cornwall.