Yep, it worked!
I also searched the registry for the LmCompatibilityLevel key and set it to a value of 2, it was originally 3. I also tried different settings for Home, Public and Work networks as I was led to believe these can affect the authentication process. It's currently set for "Home" btw.
I may return the LmCompatibilityLevel key to a value of 3, as I read that Samba4 plays much nicer with recent versions of Windows, so handling NTLMv2 shouldn't be a problem.
Regardless, I will make one change at a time and report back with what I find!
Thanks again for the 2 pennies, well worth it imo!!