Right you are. It was a bit more involved than I had expected. Not sure what I did to trigger this, but maybe the fact that I pulled the failed drive and inserted the new one in the slot the failing one was in, caused some confusion.
I had to stop the array, remove the drive from the array, tried to stop the array again but it couldn't unmount the shares for whatever reason, rebooted, stopped the array, assigned the drive again, restarted array - partity check has begun.
It's now back on-line and the speeds are looking much improved.
Thanks for the assist, and apologies for thread hijacking as the mover issue is still pertinent.