It sounds like the Dalemain Estate take their job of looking after the deer herd seriously. So from their point of view, they're not considering the case of one walker; they're choosing between two policies:
1. letting anybody walk up the road and onto the Nab
2. only allowing access through the Access Land
Policy 1 would end up with much more disturbance to the deer, so they're following policy 2 instead. I don't see a problem with that - and to be honest, if you were "aware that there are access issues" then I don't think you should have planned that route in the first place.
Fair enough - though as I said in my OP I had scanned my route and I was he only person in the valley. I had also tried to contact the estate. Interestingly I was on the Nab on Saturday and I came across three groups of deer - very beautiful. I was careful to move slowly and try to avoid them, and they seemed pretty chilled about my presence.
Mel - couldn't agree more - I think I spend nearly as much time on paths not marked on maps (or off piste entirely) than I do on marked ones.
Paul