I totally agree, even the footpath on the bridge was awful. Whilst I am not advocating confrontation, the only way to deal with this is not dog wardens (they are never about when required) or CCTV, (culprits maybe seen but cannot be traced) it's reasonable people politely pointing out to errant dog walkers the error of thier ways.
I, for one, will be carrying a pocket camera with me on my walks in future, as a last resort, to photograph both offenders and fouling. I am fed up taking our visitors and guests, not to mention our 6 year old Goddaughter, for walks up to our knees in dog doggy doos.