Didn't realize the car park thing was such an established mistake, reassuring to hear that! As forgotmyoldpassword and windyrigg suggest, I think I may have attached myself too closely (at that point) to my map, and not enough to the compass.
ninthface, you're absolutely right, I see it's marked as a path, not a recreational route. The second half of my planned route was down the Penine Way from Black Hill (which is marked on the map as a recreational route), and at some point I must have conflated that section with the section I was on. All part of the learning process...
You said the missing path may be 'one of the figments of the Definitive Map' - does that mean it mean just be a historical holder, e.g. possibly from before the quarry itself in use? If not, why would a ROW be established up the side of a quarry? I'd actually read in Navigation for Walkers before I set off that I should be wary of green lines not accompanied by black lines, but it must have slipped from my mind - definitely adding that pointer to my mental store now.
Also, can someone recommend a resource (ideally a website) which can help with map notation when planning a route? I don't know if there's a standard system, but I was reading somewhere about triangles with a dot in them vs. circles with a dot in them etc. I freestyled my own notation when planning this last route, but it's a bit of a mess and very inconsistent.