Simple. I have set a static IP address to my MyBook Live and the DNS as being the router as it also acts as a DNS proxy.
I’ve now had a look at the network settings and the MyBook Live has changed these to the public DNS server IP addresses that the router knows.
Why did the MyBook Live change it’s static DNS entries WITHOUT my permission and desire?
(I’ve removed the third DNS entry. Let’s see if it reappears.)