cheers, got it
just needed a little pointer.
It was the soft mount option in fstab i was looking for...
soft: If an NFS file operation has a major timeout then report an I/O error to the calling program. The default is to continue retrying NFS file operations indefinitely.
Using that lets me umount the share, although I see it's not recommended, hence isn't the default. none of the timeout options seem to apply though so it'll have to do.