Hi guys,
Thanks for all of the suggestions and apologies for the slow reply - only way around it (I had reached my maximum talent level) was to reinstall both machines. They were only 'vanilla' ubuntu jaunty installs so it was fairly simple.
The reason I went for NFS over Samba was how forehead-slappingly simple it was to set up the first time I used it. Export the directory from one machine, mount it on the other, it's just done. This time is just seemed to completely screw the client machine up.
However, you'll all be pleased to know that all now seems OK after a reinstall.