I asked the vets at work about your duck and chicken dilemma. There should be no problems at all keeping them both together and chickens alone run the risk of salmonella without the ducks, that's why we are advised to cook our eggs properly. If you do have any concerns then you can have the chicken faeces tested for Salmonella bacteria by collecting samples and taking them along to your vets. They advise 10 samples per 100 chickens for the best results and a minimum of 5 for lesser numbers. However, there would be a fee.
thanks for your help cazi youve put my mind at rest, scrambled eggs for breakfast now, cant wait till the ducks start laying too.