Your home buildings insurance may cover a lot of things, but it's unlikely to cover your building utilities. This means your plumbing and pipes, your boiler and central heating, your electricity and gas supply. If something goes wrong with these, you'll need seperate arrangements, or may end up forking out a lot for repairs.
Home emergency insurace covers you for repairs to these parts of your home, as well as some other risks, like pest infestations.
For full details on our home emergency cover, you can read the relevant section of the policy document linked to on our product information page.