I've now rebooted to check, and /etc/resolv.conf has again been
overwritten by NM, despite the PEERDNS=no line in the ifcfg file, i.e.
it has reverted to what it was. NM seems to be calling dhclient with its
own private config file, the whereabouts of which are non-obvious.
Strange. You might then need to set the DNS server through the NM GUI.