Well put Martyn - think we're making a mountain out of waste heap with this one........
My approach to access is simple. If it looks like open moorland/hill then I access it (unless there are specific signs prohibiting pedestrian access or well established access issues that have been widely published).
If it is fenced off (rural farmland etc.) but without any pedestrian access prohibition signs, then I try to find someone to ask locally; farmer; "someone on the gate"; etc. I NEVER start writing to head office.
If I can't find anyone and there are no obvious health an safety issues (it's marked as a shooting range/minefield) and I can access without damaging boundary fences or walls then I wander up "bold as brass". If someone was to ask me to leave, I would leave, explaining that I tried to find someone to ask and say sorry. Simple.
I'm safe in the knowledge that I've made practical efforts not to upset anyone. That's good enough for me.
It's stood me in good stead for 40 years in the UK. Of course attitudes in other countries vary, I wouldn't do this on a Ranch in North Dakota (apologies if I am maligning the good landowners of North Dakota).
It's simple really - use your common sense.