Change of plans, going to rip the head off, the machinist has been around and had a look, and seems to think the head is warped, but not cracked.
However... After a big effort by myself, my engine reconditioner mate and another R31 club member, we managed to snap 3 head bolts off, discover a rounded off manifold bolt, an inaccessable manifold bolt, remove the bottom shield with side cutters, get completely filthy, lose the throttle linkage split pin, lose several bolts and nuts (should be able to find them) bang innumerable fingers, make a huge mess, get wet, pull off a few hoses, become completely jealous of small Asian fingers, pull off a heat shield, and get a lot of crap in my eyes and mouth. AND THE HEAD IS STILL ON THE DAMN THING! Time to get a tow to have a professional rip the head off. I'll gladly pay for someone else to get crap in their face just for the pleasure of not having climb under it again. Also thinking of getting that same professional to install the head afterwards, as I'm going to run in to some of the same problems when re-assembling it, and they look to be a whole lot harder trying to put them back (hard to reach bolts and such, which could be undone in barbaric and strange ways, but will have to go back in in a much nicer manner).
It shouldn't be this hard!
Cheers for the advice though.