Unknown duck

Unknown duck

Observation - Unknown duck - UK and Ireland. Description: Seen on reservoir with a large group of Mallards. They seem slightly larger than the Mallards. Orange feet, and green head. One had white markings on wing feathers but not the other (as in photo). Help with ID would be appreciated, as I can't find the

Seen on reservoir with a large group of Mallards. They seem slightly larger than the Mallards. Orange feet, and green head. One had white markings on wing feathers but not the other (as in photo). Help with ID would be appreciated, as I can't find them in my Mitchell Beasley nor online RSPB identifier.