Ofwat have released a large number of useful reports – check these out on their website

http://www.ofwat.gov.uk/publications/

These span many years but at least it enables access – they include:

What is cost of reducing ammonia, nitrates and BOD in sewage treatment works effluent?

http://www.ofwat.gov.uk/publication/wp-contentuploads201511pap_rsh_opa2004-05-pdf/

Changes in frequencies of extreme rainfall events for selected towns and cities (2010)

http://www.ofwat.gov.uk/publication/changes-in-the-frequency-of-extreme-rainfall-events-for-selected-towns-and-cities/