ok a quick update. we have just come back from the vets with a vitamin supplement - spent 2 hours - but got it. Got home to find duckling number 3 has died and a fourth is looking poorly.
As for bedding they were on shredded paper, which we changed when the first one died, thinking that may be the cause. Changed it to shavings from our local agricultural supplier. duckling number 2 died, so we changed over to straw. Its so hard to lose them when you have watched them hatch and even more so when we have just hatched 2 goslings, whose mother has died so we so desperately dont want the same to happen to those.
We are British and live in France and have not been able to find Niacin or brewers yeast, but hope what the vet has given will help. If not he wants us to take a dead duckling in to perform and autopsy.
Thank you all so much for your help, advice and thoughts, you are all so very kind.
We will keep you posted.