RENUP on the money
Too Dogs, the AD operator is advising that the Obstacle free gradient has been surveyed out to 7500m only.
When you consider it may take up to 15km to achieve the LSALT/MSA for some 20.7.1b aircraft, 7500m is only a very small area to be surveyed. The high terrain in and arround LHR is what makes this important. At aerodromes where the LSALT or MSA are relitively close to the AD height (ie flat or costal terrain) this is rarely a problem and 7500m, which is the standard, is quite suffient for surveys and operating data.
What it really says, is that if you fly 20.7.1b aircraft into LHR, you need to assure your own terrain clearnce when outside the surveyed area (7500m). This can be done by doing your own legwork from Topo maps etc, having a survey done yourself and/or incorporating a procedure in your Opsmanual to meet the terrain clearance requirments.
Richo