Have redone the manifold and flange with gasket cement now. Changed the O2 sensor at the same time.
Now I have a torque wrench it's obvious I did the bolts up WAY too tight last time. No grunting needed, in fact hardly any further exertion required once the bolt "grabbed" and I moved to tightening with a levering bar. I did them all to 27Nm.
Also made sure I didn't do up the exhaust flange bolts too hard this time (see
http://www.fordforums.com.au/showthread.php?t=11342451 ). Tightened them enough that wiggling and shoving the flange didn't move it relative to the nut, then tightened a little more.
I kept the same manifold bolts (the couple of bolt specialists I asked at don't do auto bolts, and I don't plan to remove the manifold again anytime soon), but I also used anti-seize spray.
I'm glad now that I decided to redo my bolts, they were definitely too tight... could that have eventually caused the gasket to fail?