ok If you hav'nt guessed - I'm keen to get this working :-)

update to above is as follows.

I thought I would try these steps again in case I got it wrong.

firstly I sudo gainroot

then...

1) cd home/user/MyDocs
2) tar xvf tether.tar
3) three files produced.
4) cp tether-dbus-config /etc/dbus-scripts.d
5) cp tether-dbus-controller.sh tether.sh /opt/tether
6) reboot
7) Par n900 and Nexus 7
8) select "use this connection for internet" in Nexus 7 bluetooth settings
9) run script: ./tether.sh start
10) message shows "Tethering from bnep0 to gprs0 activated"
11) Try to access web on Nexus...
12) no internet connection

Help me save my N900 for a bit longer :-) think of it like rescuing a puppy.