OK, for some reason our NFS server was 10 seconds fast and this upsets Make as files seem to have future dates.
The date command on linux really does have a crap syntax doesn't it.
I set the time back 10 seconds to correct, and it all went pear shaped.
Rebooting the NFS server did not help. The client says permission denied and the server says unsupported file system or fsid= required. WTF
Turns out this is all due to my setting the clock 41 years fast...
Really helpful messages!!!