Hello!
Figured it would be nice to give you all a short update.
In the end I drilled the holes out to 12mm, and used M10 anchor bolts (this gave me just enough slack if the drill drifted for any reason). Drilling out the metal was a pain in the proverbial but I got there in the end.
The rack is up and solid. So far I am just loading it with just under 160kg, but I'm confident in the wall and bolts so I'm sure it'll cope with the next plates I buy for it.
Thanks again for all the advice.
https://i.~ibb.co~/t42FpHZ/garage-rack.jpg