Bald Hill is not at all bald. I assume at some point it was, but it now has head-height acacias growing all over it, and from the direction of approach of the old access track it's nearly impossible to get through. I later figured out that the opposite slope is much clearer, but I was following the track and wondering why it was strewn with lumps of grey concrete so I pushed on through the scrub.
It turns out there's a very sizable concrete footing extant there, with massive reinforcing sticking out of it. There's a water tank further up the hill, and lots of red brick rubble about. I have no idea what it was all for, but I presume WWII stuff (it looks similar to other brick and concrete ruins I've come across on the island). The size of the footings certainly makes it seem likely.
I'll upload more photos here later; the only ones I have on my phone are crappy due to a scratch on the lens (which, I've just read, might be fixable). My camera works well, and I had my GPX turned on so shall have coordinates for all this too — when I'm back at the computer.
Now, it's beer o'clock.