Wow .. thanks for the quick responses!
It's good to know it's all about taste, and not some toxic chemical reaction
I did try grinding some kosher salt, but not very successfully. I needed quite a lot, as one of the olive batches was dry-packed in salt. I blunted the blade of the grinder before getting very far, and ended up using the iodised instead.
I'll try and find some pickling salt, thanks!