Feel the bottom of the hen between her vent and keel bone if she is eggbound this are may be hard and even have some fever to it. If you are unsure compare what you are feeling to the other hens that you have.
I will take a big chance here and assume you are in the UK. There is a product there called poultry spice that is a pro-biotic that many use to keep the health up on their birds. It works well for the ex-batts too. It is close to the bottom of this link.
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chickens can live longer then you think lol my favorite bantam (shown in avatar) is about 7 years old. or was it 8? and can live to be around 12!