Huge mystery grey / black ducks

Hello

I’ve been spotting these mystery birds sporadically since the summer on my local stretch of the Thames.

They’re about goose size, orange legs, yellow/grey bills. Dark grey feathers with white chests and black necks and heads. The photos show them having a sleep but when they’re stood or swimming they look like a mallard shape, just treble the size.

At first there were just the two who’d be found happily swimming around with the other mallards. In these photos (taken last week) it looks like they’ve had a chick! The third bird is dark dark brown, fluffy looking and the size of a regular mallard.

What on Earth are these lovely birds? Feeling like Charles Darwin on Galapagos right now, I’ve searched online and in every book I’ve got and can’t seem to find a name! Would appreciate any tips