Working at heights can be dangerous which is why Scaffolding is needed to access the chimney on nearly all properties.
Anyone carrying out work at heights without it wont be insured and any damage to your property wont be covered by their Public Liability Insurance (PLI).The homeowner employing the tradesperson is also responsible if they have an accident and could end up in court and prosecuted by the local authority. Always ask for proof of PLI for anyone working on your property.