I started getting these two days ago and a sniffer trace shows that
my ISP has suddenly started sending DNS responses from a different IP
address than that to which the request was sent.

ie the DHCP config response tells me to use 194.168.4.100 for DNS but
when my system sends a DNS request there the response comes back from
62.253.170.243.

Looking at what I think are the latest sources for DNRD there is a
check that compares the configured IP address to the response source
IP address and ignores the response (with the above error message) if
they dont match.

Is there supposed to be a 1:1 corespondence between the two ??

This causes problems for my firewall, but windows 98 seems happy with
this (I know that doesn't say much!).