Sorry - I forgot that what we call a Pekin Bantam in New Zealand is called a Cochin Bantam in America, so no duck eggs, just bantam chicken eggs. I did have to giggle at the thought of my tiny Old English Game Bantams trying to sit on a bunch of duck eggs! Six bantam eggs under each of them seems quite a lot, though Sunflower had stolen one of Pixie's yesterday, so I'm sure she'd be game to try sitting on some big duck eggs.
Thanks everyone for all the info and ideas though. At least I know what to do if I ever want to hatch duck eggs - but I think I'd choose a bigger broody.
They can do it... My tiny little Blacktail buff Japanese bantam Ishtar hatched 7 big Duck eggs for me over the winter.... And took care of all the babies even when they were twice her size....