So it turns out it needed a 15mm Allen key. I didn't see the splines inside as they weren't obvious so after shining a torch through there I just about found them.
Thanks for the help though. It turns out the the grit is in the inside of the freehub body itself and to your point Iride01 I'm just being finicky. The compulsion to keep it in absolute perfect condition is strong but its also very time consuming and expensive 😅