Thanks to the info I found on previous threads I have now got the heads off my GT4. I made up the puller plate but it still took a few hours to pull each head off. Several of the head studs were completely bound to the head by a white compound that looks very much like hard water residue from a kettle. My question now is what causes it and what can be done to stop it happening again. I measured the studs at 10.0mm and the holes at 11.8mm so I don't think it can be a clearance issue. Is it possible for water to seep into the stud bores and would this indicate a design issue with the head gaskets?
