Previous threads we've done: https://news.ycombinator.com/submitted?id=proberts.
However they keep flip flopping between me needing a B1 and me just using my ESTA for the training, and their communication hasn't been the most straight forward. Which visa do I need to get to enter the US for the training?
> attend short-term training (you may not be paid by any source in the United States with the exception of expenses incidental to your stay)
"source in the US" might be problematic if you're paid by the US company directly and not a UK arm. You'd have to take those days as unpaid, except for a per diem? If you're paid in pounds by a UK source, ought to be fine.
I would confidently say you do not need a C visa. That's for immediate transit (like you have to change airports or something, and you would use an ESTA anyway). A D visa is for people like airline crew who have to stay and have to work whilst there (like getting an aircraft ready for international departure from a US airport). If you needed that, your company ought to know.