London's Greenbelt - how developed is it?

While visiting a site on abandoned Underground stations http://www.starfury.demon.co.uk/uground/, I read where several planned stations were dropped because they would have ended up in the (then new) Greenbelt, and the potential traffic would be too low.
I’ve known about London’s Greenbelt (and the green belt concept in general) for years, and always thought it was a wide strip of land encircling London where development was prohibited, or at least very limited (think parks, wilderness areas, farmland, etc.)
However, while googling around the Web, I found that not only does development occur in the Greenbelt, it seems it’s fairly widespread and sometimes intense.
So, anybody out there able to clarify this for me?

The restrictions on development within the greenbelt have never been absolute and always did depend on the willingness of central government and the various local authorities to enforce them. Over the years much development has therefore been allowed to take place. That said, there are places where the edge of the London urban sprawl is still very sharply defined. Large areas of relatively undeveloped countryside can still be found close to the capital. Even somewhere such as Essex, which no one would ever describe as a rural idyll, there remain obvious pockets of agricultural activity. Without the greenbelt regulations, Greater London would undoubtedly have spread much further than it does. What, of course, this means is that the problem has been pushed out beyond the greenbelt, with many commuters being forced to live even further distances from London. The result is that the problem of overdevelopment is one which now affects the whole south-east rather than just the immediate vicinity of the capital.

Thanks, APB
I had read on one of the few sites I saw (they were not very in-depth) that the whole idea was of a central core, a somewhat sparse inner ring , and then a more developed outer ring. It seems this is exactly what happened, for good or bad.
Does anyone know any good overall descriptions of the Greenbelt on-line - every time I google I get descriptions of Greenbelt Arts Festivals, Greenbelt School districts, and other stuff but no one good site.

Basically, I think you’d want to add terms such as “planning” to your search - that will help dig up stuff from Town & Country Planning Acts.

And look at http://www.detr.gov.uk/
(Department of Transport, Local Government & Regions)

and perhaps http://www.cpre.org.uk/main.htm
(Council for the Portection of Rural England)

Good luck