Problem automatically unmounting VPN shares

[Peter Rockett <p rockett sheffield ac uk>]

Hi

I am trying to automatically mount and unmount a share at my office. I 
have a script in /etc/NetworkManager/dispatch.d which runs along the lines:

#!/bin/bash

case "$2" in
    vpn-up)
        exec mount blah-blah-blah
        ;;
    vpn-down)
        exec umount blah-blah-blah
        ;;
esac

which mounts the share in my home directory (on Ubuntu 11.10 + Network 
Manager v. 0.9.1.90). I swear this all worked fine when I set it up in 
2010. But some time in the intervening period it has broken. (I am 
infrequent user of VPN since I got DropBox!) I have also upgraded from 
Natty to Oneiric in the meantime...

Now, VPN connects fine, the remote share is mounted OK. I can see/access 
all the remote files. But when I disconnect from VPN, the running of the 
umount seems to go very wrong. After disconnect, if I open a terminal 
and type "ls" to list the contents of my home directory (the mount 
point), the "ls" command hangs. (Nautilus also hangs - either it won't 
start or, if it was running already, it hangs.) Everything else in the 
OS seems to work OK. It seems for all the world as if the mount point is 
getting screwed and the only recovery is to restart (in which the 
shutdown takes much longer than usual).

The weird thing is that, if I delete the dispatcher script above, 
connect to VPN, mount the drive manually in a terminal, then umount it 
manually, and disconnect from VPN, everything is fine and as it should 
be. So it seems like the dispatcher daemon is making a mess of running 
the unmount... but not the mount.

Any ideas where to go with this?

Peter