hi and welcome. this is a very common problem with cats, and the most important thing you can do is support the immune system so they are able to fight it off themselves. I will give you a link to a posting I just made recently on the same subject, I see you are in the UK, don't know if iherb will ship to the UK? but I do know they do ship internationally b/c I get mine shipped here to Canada, if not that try and find the same products even if its a different brands, as long as they are pure without additives such as sugar, salt, soy, yeast, preservatives etc....you can buy at a human health/vitamin shop.
good luck♥
http://www.medhelp.org/posts/Cats/sneezing-with-discharge/show/1875218