If the easy out hasn't worked, another thing I have seen in workshops but never used is a spray that freezes the bolt, it's used to extract bolts from engines. The theory is you spray the bolt and the contraction cracks it away from the thread
Jeff Southall Current boats Nacra 5.8 1703 Animal Scanning Services Nacra 5.8 1667 Ram Raider Nacra 18 Square Arrow 1576