Did they give you any notice or was it an emergency job?
Some statutory undertakers eg gas, electricity and water suppliers have certains rights of access over part of pproperty where their cabling, pipes and similar run underneath.
Usually it would come up when you bought on the Water and Drainage search or, if a new build, s.104 Agreements or the lease/transfer.
Speak to the water board and ask them to explain the circumstances such as their right to enter and do work without notice ie was it an emergency and where they have their authority from and if there is any source of redress.