Cyngor Gwynedd does not operate a service for private householders to clear private sewers and drains.
You will need to arrange this work yourself by contacting a private contractor. You can search online (for example, using Google or Bing) for local companies using terms such as “drain cleaning” or “drain and pipe cleaning”, or consult trade directories such as the Yellow Pages.
Please note that you are responsible for arranging and covering the cost of these services.