Two bolts. Looks like this:
http://www.urotuning.com/shop/cooling-hose-flange-p-949.htmlIf it is this and you end up doing it, it's not a difficult job, just takes a bit of time and the hose clamps can be a fiddle. If you aint got the proper tool for them, I've found mole-grips set just right are good for holding the clamp open and moving it off the fitting.
When you take the old housing off, the aluminium head may be scored where the coolant has been leaking past, so clean this up properly with some wet&dry back to a nice flat surface. Apply a small amount of grease to the o-ring prior to fitment to ensure good contact with the head.
If you haven't already, I'd suggest you also replace the coolant temp sensor (and o-ring) if you are taking it all off and draining the coolant.