I had a S1 Manual VN a while ago and had a loose water pipe into the thermo housing. I bought a new one straight away and f#%@&ed around for quite some time to get the (single) bolt out of the housing.
After this VN parted the family, we bought a series 1 wagon and not long after we bought it, I needed to do a gasket overhaul as it was losing water into the inlets from the manifold "Valley" gasket
My suggestion would be to get yourself a bit of gasket paper, for the throttle body, (just incase of course) and remove the throttle body. This is quite simple, even the amatuer could handle it. this should give you a good shot at the thermostat housing too.
make sure the O Ring that seals the Thermostat housing is in servicable condition too before installing the housing again and give it a small smear of some form of rubber lubricant so it slides into place nicely without being dislodged.
The best thing Holden ever did, IMO, is relocate this to the front of the motor.
Hope this is of a little assistance