Thanks Fowlman I hadn't thought about looking at the acidity, so have gone and done a quick check and the acidity of most shop bought cider vinegars is around 5%, which is comparable with red wine vinegar, white wine vinegar and malt vinegar.
For anybody intersted in the science behind vinegar I found this site http://www.versatilevinegar.org/faqs.html
so it appears that the additional benefits seem to be related to the nutrients that are imparted by the source material, which if I am giving them a good varied diet should be available anyway.
So i am going to do a test and give one lot of my hens cider vinegar and the others malt vinegar, then i will get OH to score them on how healthy they look, egg yield , energy levels...anything else I can think off. i wont tell him which have had which vinegar and see if there is any difference. I guess if I run this for a fortnight we should be able to see any difference....I suppose to be totally fair I should have a third group who don't get any vinegar...but I don't have enough space to be able to split the flock into 3.