Clee,
the CAA designated PSZ was changed about 10 years ago from a fan shape spreading out from the end of a runway to a triangle with its base extending either side of the centreline and coming to an apex as it extends from the end of the runway. For example LHR has a PSZ designation of both 27L and 27R which extend from a base 350m either side of centreline to an apex at 3.5Km.
Th PSZ is based on modelling of aircraft accidents since WW2 apparently - i.e. the further you get from the end of the runway the less likley you are to crash and, if you do, the more likley that you will be close to the centreline.
On this basis I would agree that within the perimeter there seems to be a very srong arguement that ground is free of non moveable obstacles which could potentially create a more dangerous situation for aircraft in an runway excursion. This would include large gullies and ravines.
The airport operators must undertake the risk assessment and should justify their decision not to undertake additional works that may be deemed reasonable. The cost is actually passed on to the users through airfield charges - so the passenger pays eventually!
Going back to LHR its not just Bath Road if you apply the Spanair circumstances to 09L you would find yourself either in the fireground or the Virgin, BM of BA hangar complex - not good!