| Steve I would try drilling right through the CENTRE first with about a 4mm drill, then use around a 6.4mm drill to take off the screw heads, then with the heat sheild removed spray on some plus gas or similar and let it soak for a while. Then try the easy outs, if they don't work drill out the hole which is hopefully central with a 5.1mm drill and dig out the screw threads from the captive nut threads with a scriber. | 
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