Steven
Thanks for your reply - I think you've given me the nudge or clue that I was seeking. By viewing the problem in space-time I can see that at the event horizon the past light-cone of an observer would be outside the event horizon while the future light cone will lie within and the light-like edge boundary of the light cones will lie on the event horizon.
Your post led me onto this page - and both have helped considerably.
http://www.phy.syr.edu/courses/modul...arzschild.html
I can't say that I immediately understand the tipping inwards of the light cone - but at least I have some handle to the answer.
Regards
Mark C.