Zig-Zag Hill! Yes, that rings a bell! But also…
Sidevalve:
They are advisory. Below is one of the reasons.From personal experience; if you are hauling a tanker, or indeed anything on or close to full weight, DO NOT under any circumstances go any other route than the A350, particularly if you are heading for Poole.
You WILL lose traction on the climb out of Dinah’s Hollow, even if the road is only slightly damp. I don’t care how good you are, I don’t care how good your truck is.
You are also wise to avoid that route if you’re driving a long wheel base rigid.
…my little…erm…experience-gaining opportunity 
…was a perfect demonstration of that! 