worms

On return from holiday, I found one very thin, poorly chicken with worms (visible in droppings). She is still and does not eat or drink. I give my 8 warrens regular herbal worm protection but since they have not been moved from their (largish) run since they arrived 2 years ago, this may not have protected them enough. Is there any point in giving her a strong wormer at this stage?
And what wormer do people use?

Thank you for your quick response. I have ordered Flubenvet online and we are due to move the chickens to a new run as soon as I get enough people to help with moving the chicken house which is very heavy.
I thought I'd mention that I found  the 2.5% strength athttp://www.hyperdrug.co.uk/Poultry-Wormers/products/449/  because it seems to be hard to find online.

I'm loving these poultry pages!