Back to Ian's original questions, none of which seem to have been answered yet.
Maybe the course length is (was) measured by driving around in the left hand lane only, using some sort of calibrated surveying equipment (not to become airborne in the usual places)!!!
What ever the true length of the course, it is the same for all competitors, so only the actual recorded average speeds may be suspect, if compared with previous years.
As Ian pointed out, to compare records, now that everybody is so much faster than say, 25 years ago, the track length needs to accurately recorded, regularly checked and adjusted if necessary. Correction factors may then be applied to effect comparisons. For the modern speeds attained, there would probably be only micro seconds in it but this might mean a new record or not!
Does anyone out there know?