Where possible use a tap wrench to insert the easy out into the drilled hole of the stud as this gives good tactile feedback when unscrewing the stud.  Use freeze and release on the stud to be removed with the easy out inserted and tightened ready for turning.  Slowly turn the easy out and eventually the stud starts to turn - STOP - more freeze and release - wait 20 seconds and start turning again after approximately one complete turn stop and apply some more freeze and release.  Continue turning the tap wrench and the stud will become free and easy to turn and can be screwed out.  I think the reason for the initial tight start is due to the damage on the leading part of the thread where the stud sheared - all of mine were distorted at the point of fracture and would cause the thread to become tight when turned.

 

Repeat for the remaining broken studs.  

 

The only difference was with the rear stud where it was not possible to use the tap wrench due to lack of space.  I resorted to using mole grips to grip the easy out - this is risky you have far less control over maintaining the square angle of the easy out which could cause the easy out to break. If this method is used take your time and do not rush it.

 

Fitting the replacement studs

Before fitting the new studs I would suggest removing any remains of the old exhaust gasket and carbon which remain on the side of the cylinder head around the exhaust ports.   I used a home made soft aluminium scrapper to do this and cleaned with WD40.

 

The new studs were easy to fit as none of the threads were damaged.

 

Good luck - just be patient and take you time.

 

Replacement studs, washers, gaskets etc for the engine were obtained from Conceptua Tuning - thanks Chris for all your assistance and super fast delivery.