It's mainly present (acutely) just after a big storm when the water company opens up its valves ! When there hasn't been a really big storm, with high levels of run off, then there is no fresh sewage present...
If you are thinking of a trip, ring one of the forest cavers, or drop Andy Clark a line, to find out current conditions...
It's interesting that they are allowed to do this (and we have tried to stop it) - after all it all enters the Wye after a couple of kilometres right into a very popular tourist (bathing) spot !!!