After a lot of research on the Internet I thought I'd write about the routine I've developed so far to (hopefully!) keep my lovely hens healthy.
1. Every morning after feeding them I clean the droppings out of the hen house. There is a layer of wood shavings in there so they are very easy to collect - into a bucket and onto the compost heap. This means it is clean for them every night - they are not breathing any nasty fumes. If necessary the shavings a topped up a little. Then once a month the whole thing is thoroughly cleaned...
2. Every day they get some greens of some sort. Mostly, at the moment, they get some turves from areas of grass I'm clearing in the garden. This means they get fresh grass to nibble, and bugs/worms to eat.
3. Once a month as I say everything is removed from the hen house. It is thoroughly swept, dusted with Barrier red mite powder, and layers of newspaper on floor topped with shavings.
4. Also at the beginning of the month, for 3 days, they get Vermx pellets in their food, to keep them clear of internal parasites.
5. Finally, and this is what prompted my post, one of them has the sniffles at the moment, and my researches led me to a product called Citricidal (grapefruit seed extract) According to the people at Omlet "Extract of grapefruit seed has been tested and validated by the FDA and Pasteur Institute in France it has been used for many years as a natural anti-viral to cure colds. You simply add one or two drops to a pint of water." So I started this morning - 4 drops to 1 litre to fill each drinker... and we'll see how they get on with that.
Updating to say that the sneezes have now disappeared and all 4 are doing great!