Whilst asking the question, I had a thought - how does it know the table schema?
Dave.
I remembered in the RDD that it automatically populated with "Erp" - so this made me believe it was accessing the tables, and I was stuck in the tables thinking how do I access the table value through a join in the F(x) query.
This lead me to take a step back and think, what's the best way of cumulating a large amount of data, then sending only what I want to see? A view.
So I looked in the views table, and low and behold, there is a dbo.QuoteHed. Expanding the columns list shows my extra UD field that I defined in the RDD.
Simply modifying the existing F(x) query inside the original Quote dataset allowed me to simply put in the extra column, using alias.Column without having to add in yet another join.
Fully available in SSRS now. So happy I figured this one out, haha.

Cheers,
Dave.