I asked the original question as I couldn't find the definitions in the Jepps and and haven't seen my perf A notes in years! Some interesting comments and info so thanks to all of you for your input. Since I started the thread I've done some more reading up and the link to the Dutch study was particularly interesting.
The QRH on the 744 has a note that states the assumption are 50 ft above the threshold and approximately 1000 ft flare distance. in that sense the 'glide slope distance' will be conservative by about 1000 ft so over cautious perhaps. Quite why Jepps do not always provide a threshold distance eg LHR 27L I still cannot determine - especially as the geometry appears identical to 27R.
What is/has always been obvious is that the data in a QRH/performance manual derived from a company test pilot flying said type may not be achievable on the line so maybe some healthy factoring is prudent. For example, using the above example a senior trainer mentioned that he believed Boeing were due to revise the tables and up the flare distance to about 1500 feet. I did notice that value (1500) mentioned in the DLR report.
Anyway, it's been a useful exercise for me to reacquaint myself with the assumptions used in the performance data. Now of course if we begin to look into landing on contaminated runways all bets are off. Best advice given to me is don't! Happy landings.
regards
BBK