I can indeed confirm there was a man and a woman with a speed gun and clipboard stationed halfway down the path last week (like a number of people here, it's part of my regular commute route).
Over and above the notion that (during the summer months at least), the path is regularly crossed by walkers, children, pets, etc, I never really hack it along there. 8mph can easily be achived and exceeded on that patch of path but one wonders how such a speed limit can be enforced without some kind of permanent presence.
That said, most of the cyclists I see keep to a respectable speed there so I can't understand the need. It's also worth noting that for every fool out of the saddle, with their head down tickling the 20+mph mark, there is an equally stupid pedestrian determined to walk along the cycle route when there is a perfectly good pedestrian path less than 5 yards away that runs parallel. *cue evil stares all around*