I haven't done the Rotsea road, so I don't know. I just had a look at Google Street View and the white road part looks suspiciously like a private farm access.
Agree. Googling for "Rotsea Carr Farm" also brings up some horror stories about animal cruelty so I think I'll give it a wide berth.
Looks like either I cycle approx 3km of grassy riverbank or I go around the road via Leven and Brandesburton. Still best I know now.