"But officials from Thames Water confirmed that following an investigation, they believe that the unknown man entered via an exit of the surface water system on a new housing estate, about a mile away. A spokesman said an engineer checked but could not find any broken manholes that he could have fallen through, following a call from police. The system carries rainwater from roads straight out to the River Rom so the explorer had not come into contact with toilet waste during his journey."
I suspect 3 days is a bit of journalistic licence.;D