Managed to resolve this issue.
I had not added an Ethernet interface.
I had assumed that as it was using DHCP to get an IP address from my router and I could connect via the web interface, and I could also SSH into the Pi, then all was well.
Now I've added an network interface resolve.conf is showing the correct name server.
root@nas:~# cat /etc/resolv.conf
nameserver 1.1.1.1
nameserver 192.168.1.254
search home
I wasn't able to add a wifi interface, as none were available. This is not a long term issue, as it is obviously better to use hard wired ethernet for a NAS, but I'm surprised it wasn't available.