Anyway, it turns out that the set was in fact a real slope up in the Midlevels.The film makers took advantage of the recently demolished Queen's Garden building at 9 Old Peak Road and used the slope behind the empty plot to stage the disaster scenes.
Before you get too excited, the slope no longer exists. Just after filming finished work began on construction of not just the new Queen's Garden but also another development behind it called Dynasty Court. The slope was removed and a podium was constructed on which Dynasty Court now stands. The driveway of Queen's garden covers the area where the cars are in the above screengrabs.
Anyway, a big thanks to Sean for letting me watch his excellent home movies and helping identify a location which I had never really considered possible to find. You can click on the location link below to see the exact spot.