Thanks, my understanding is much improved.
The actual cable run is along a main road that goes back a century or two, with well spread out houses along its path, before it reaches my plot. So I am guessing the DP will be somewhere in the middle of that cable run, which would explain the optimistic speed estimates.
Most of the neighbouring houses, including mine and others 'downstream' are fairly new, and probably built long after the location for DP was chosen. Most of the wiring is indeed underground, although a few poles exist to supply flying wires to individual houses.
One of the holes in my garden is about 20 x 10 inches, and was installed when a pole was removed, a few years ago. I got a peek inside recently when an engineer was checking it and I suspect it contains just the cylindrical junction thingie that used to be atop the pole. The pole used to have flying wires to another house, but they now fly from a different pole, which was planted at the same time.
The larger hole, which follows after about 100 ft in an underground duct, is a more substantial manhole cover, about 30x15, and I think has been there a long time. I've never seen inside it, but I suspect that is where most of the neighbours' lines are teed off.
