Thanks Craig, Russell and Trent for the responses.
you can use rpcinfo to find out what versions of NFS are available on your NAS.
rpcinfo -u YOUR-NAS-SERVER-IP nfs
e.g. you'd see something like this:
# rpcinfo -u localhost nfs program 100003 version 2 ready and waiting program 100003 version 3 ready and waiting program 100003 version 4 ready and waiting This is exactly what I got
if nfs v4 is available, then changing to it is probably as simple as unmounting the NFS mount, editing /etc/fstab to change 'nfs' to 'nfs4' and then remounting the share.
NOTE HOWEVER, that this may not fix the problem....but it's worth a try if anything else needs to lock files on the NFS server, and especially if it's shared with multiple simultaneous users.
Using nfs4 is the fstab entry didn't work - the share wasn't mounted.
it links to another post on ubuntuforums.org, which suggests either a) mounting the NFS share with the 'nolock' option, to disable locking
The 'nolock' solution works perfectly for me. I can now access documents OK Thanks again Bob