And that works... Quite why *beats will change the path.config according to the path or no path specified on the command line I have no idea... But thanks for that. It would be nice if it didn't do that, but I assume there's reason there somewhere.
I know why I was specifying the path though... The install docs on the download page specify a path... Which will never work BTW because the command
sudo ./metricbeats -e -c metricbeats.yml
doesn't get the paths right either... And assumes you've done a 'cd /usr/share/metricbeats/bin' before it (Which isn't specified). And assumes the yml is in the same directory as the binary... Which it isn't...