Anecdotal evidence only, but I noticed improvements in vigor, yield and health of the plants when I followed coots advice and added rock dusts. My first couple batches of soil went without it because I was still focusing solely on NPK. Subsequent soils had the rock dusts included and I could see the benefits, particularly when you get in to the 2'nd and 3'rd runs of that same bucket .
If you have a rock quarry close by you could take a 5 gallon bucket up there and ask the owner if he'd mind giving/selling you some. Places that carve headstones, or custom install granite counter tops are also good places to check with. If worse comes to worse you can order it online. There's a place in the states here called rock dust local that I ordered from before stumbling on a rock quarry close to my house. I want to say I paid $70'ish for the stuff online and it lasted quite a while. 80lbs worth for that price, and you get roughly 3 cups per lb. That would give you enough to amend 80cf of soil.