There are a number of petitions hosted at the Prime Minister's Office and I recommend checking them out. After all, politicians seem to respond more than ever to strong public opinion.
I particulrly recommend Paul Roberts petition: "We the undersigned petition the Prime Minister to legislate that all sewage is treated to tertiary level prior to discharge into the sea." follow this link and add your support: [petitions.pm.gov.uk]
Please also forward this link to others who will be interested and ask them to do the same.