" Seeing as no one has attempted to answer my question as to exactly where along the runway the glideslope is targeted...
I'll ask it again, please....?"
From the AIP: 27L PAPI 420M in from THR, MEHT 65ft.
So, the PAPI (roughly in the middle of the touchdown zone, TDZ), is inset 420 metres from the painted threshold (not necessarily the end of the tarmac). The glidepath will "aim" at the same point, being calibrated to achieve this. The MEHT (minimum eye height threshold) is 65ft, which means that the pilot's eyes must be at least 65ft above ground as he crosses the threshold to see an "on glide path" indication.
If he is doing an autloand, the MEHT and the PAPI may not be relevant, as he possibly can't see them (fog - although not in this case before I get flamed
), and the autoland system will take care of the flare, landing and possibly rollout. If however, he is flying the aircraft visually, the PAPI will provide a visual reference of his position on the glide path. Bear in mind that in a larger aircraft, the pilot is higher in the airframe, so as his eyes cross the threshold at the MEHT (or hopefully, somewhere above the MEHT), the wheels will be considerable lower, so a larger aircraft will touchdown earlier than a smaller one, given that all other factors are equal. The MEHT is calculated to take this into account.
See CAP637, available from the CAA, for further definitions, and the AIP for all the numbers.
Not sure what you're going to prove now you've got the numbers though...