Whilst it may well be the case that many road race courses were not that accurately measured in the 80s (and 70s), isn't it also the case that guys in the 80s just trained a lot harder (and with greater abandon back then). It was the 'throw the eggs against the well theory'. If you didn't break, you'd adapt to the stressors and run very well.
Some posters on this thread have stated that there were 200+ sub-2:20 marathoners in a random year in the 80s. Stating such stats adds nothing to this discussion, since the length of those marathons is the very thing that is under a cloud of suspicion.
But the great depth in 5K and 10K running in the '80s would tend to lend support to the theory that the recent proliferation of USATF-certified courses in the last 15 years cannot, or should not, account for the decline in performances.
I have at my disposal a decade or so of Athletics Weekly magazines. I picked up the first issue I found that had the UK rankings (1985). The drop off in depth over this 21-year period is quite staggering.
A comparison of 1985 and 2006 UK rankings:
3000m S/C
Time..........Athlete...................Rank....Time........Athlete
8:13.50....Colin Reitz.................1.......8:28.43....Adam Bowden
8:34.77....Kevin Capper.............5.......8:43.86...Luke Gunn
8:46.97....Nick Peach................10......8:47.86...James Bailey
8:55.67....Gordon Russell.........20......9:05.02...Chris Warburton
9:00.0......Geoff Wightman.......30......9:18.28...Adrian Holliday
10,000m
Time..........Athlete...................Rank.....Time.........Athlete
27:53.91...Steve Jones................1.......28:32.92...Andrew Lemoncello
28:42.01...Mark Scrutton............5.......29:41.77...Gavin Thompson
29:05.9.....Martin McLoughlin....10......30:07.11...Richard Kay
29:20.6.....Graham Smith...........20......30:50.16...Neil Burton
29:36.42...Mike McLeod.............30......31:42.04...Louis Jones