This is how it is these days.
Southbound use the Melbury Abbas/Dinahs Hollow/Spreadeagle Hill/High Road.
Northbound - use A350.
Big dot matrix signs show you the way.
The Zig Zag is nothing to do with this - it’s a different road entirely, although nearby. And not really recommended for anything bigger than 7.5t although short wheelbase 18t are OK. It’s improved a bit recently - new road surface and kerbage.
Cue posters telling me they have taken tank transporters and trombone trailers down (and up) this incline. 