Welcome from the UK. No doubt you will eventually read about Diatomaeceous earth (sp?) (DE) which is a chalk dust you can buy and feed to your chickens and dust them with it. Its razor sharp at cellular level and cleans the mites/lice/worms up by slicing their skins and they lose their inner fluids and die. Read up on it and use it regularly - it doesn't harm the chickens at all. Also, putting apple cider vinegar in their water (1 tbsp/gallon and use plastic dispensers, not metal) gives them some protection from pests and helps keep their systems in good shape.