I'm an insurance broker and may be able to help if I know a bit more about the situation. In essence, this sort of insurance would cover you for liability to the public if you do something which causes damage to someone or something whiclts you are working. It very much depends on what trade you are in, whether you're a company, sole trader etc.
If you need help let me know.
A good simple explanation of insurance needs can be found on
www.businesslink.gov.uk
search the site for insurance and you should find what you need