This has bugged me for a while now about safari on the iPad, I just found out how to do it ... You need to use 2 fingers to scroll within the scroll part of the screen, if you want to continue scrolling as long as you keep one finger on the screen you can continue to scroll up and down the page ...
Hope that helps
Aj .... Just re-read your post and realised what you said was the same thing, I was originally holding one finger outside the scroll part