Clearing a blocked sewer can often be achieved by simply rodding the system with a series of drain rods.
If no one has access to a set of rods or the blockage cannot be cleared by other means, then it is recommended that you contact a drainage contractor, details of which can be found in the yellow pages. These companies have specialist equipment (e.g. water jets) that can be used to clear your system. If you decide to clear the system yourself then adequate precautions should be taken (e.g. gloves, washing hands etc.) to prevent any possibility of ingesting the foul material.
If you think there is a problem in the public sewer then telephone the relevant water company.
For Anglian Water, telephone 0845 7145 145
For Severn Trent, telephone 0800 783 44444