Nevermind, I solved it and it doesn't involve the synology.

I assumed the permissions were just mirrored across the host and container, but they're not, you can change the permissions on the container side quite happily to 777 an it can make use of the device, the permissions don't change on the host side.

Just need to modify my docker file to add in a script to do this at boot time and it should all work straight away after a reboot.