Public Liability cover protects you in the event of a claim from a member of the public whether it is in injury, property damage and associated financial loss. Any business that has any interaction with the public should have Public Liability cover.
Whether you are a sole trader or a large corporate it is difficult to see how you would not deal with members of the public.
Insurers base their premium on a number of factors however initially they will consider the description of your business and what you do. It is important that the information you give them incorporates all your activities, both at your premises and away from your premises. Otherwise Insurers will not settle any claims for activities outside this definition.
For example if you are a Restaurant are you involved in Outside Catering?
Some policies extend to include Wrongful Arrest which may prove important for a Shop or similar business.
The level of cover given for Public Liability is subject to the Limit of Indemnity agreed on between you, the client, and Insurers.
Liability arising out of advice or from faulty specification would not be covered under this policy and would warrant Professional Indemnity cover.