Well it happened. I was able to ping 4.2.2.1 OK, I was able to reach PPRuNe by using the IP address, so the next thing to try is to enter manual DNS direct on one of the PC's , so next time I'll see if that one is unaffected.
I have IPv6 disabled on the Llinux machine already BTW.
There is definitely a new router in my near future!