Public Liability Insurance
Public Liability Insurance ensures that there are sufficient funds available to pay compensation in the event of personal injury or damage to property - it does not protect against bad workmanship.
The insurance policy is owned by the trader. Just like any other insurance policy, the trader is able to set an insurance 'excess'. The bigger the 'excess', the lower the premiums paid by the trader will be.
You cannot claim on a traders public liability insurance. Only the trader can make a claim. This means that you would need to take a trader to court, prove they are at fault and have compensation awarded to you. If they have insurance, then the insurance company will provide the funds to pay that compensation less the insurance 'excess' which will need to be paid by the trader themselves. Clearly, being insured is not a quick or easy fix to any problem that arises.
Public Liability Insurance is normally specific to a particular trade as different trades have different levels of risk. For example, tree surgery carries a higher risk than lawn mowing. Someone who specialises in one trade is more likely to have appropriate insurance. Just because someone is insured doesnt mean they are covered for every activity. Remember, if the trader is proven to be liable then you are likely to be awarded compensation. Insurance ensures that the trader can afford to pay that compensation without having to sell their house, their business, take out a loan etc to pay it. For this reason, it is in the traders interests to have insurance, they would be stupid not to be insured. They would also be stupid to conduct activities that were not covered by their insurance policy.
Accidents will happen. Reduce risk by hiring a professional who is more likely to stick to the activities they are insured for. Don't be tempted to ask them to do a task you know a different specialist should do. Avoid risk of injury by keeping your distance when your gardener is working. If you are concerned, ask to see the traders public liability insurance certificate to see what is actually covered.
Professional Indemnity Insurance
Some gardeners offer a consultancy service. Where a gardener gives advice only, their Public Liability Insurance will not cover any financial losses arising from their advice. Gardeners who focus on consultancy are more likely to have Professional Indemnity Insurance or both.