When removing corroded old plug ferrules in the past, I have firstly taken a hacksaw blade, wrapped one end with tape to make a handle, and then VERY SLOWLY AND CAREFULLY cut two slots, at 180 degrees to one another down the internal threads. It is a tricky thing to do, as every few cuts you have to check that you haven’t gone too deep and into the threads of the cylinder head. I check by shining a strong light down the cuts, and at the stage when you have got to the bottom of the threads, a row of pinholes will appear at the bottom of the saw cut. Obviously you must stop there ! With the two cuts made some penetrating fluid into them will seep into the threaded joint, and when the ferrule is gripped tightly it will partially collapse into the saw cuts, and then unscrew quite easily. I didn’t detail this procedure in my previous posting as I could foresee people going too far with the hacksaw blade and damaging the threads in the cylinder heads. I cannot overemphasise how carefully the two cuts have to be made, and I have only now explained it for the benefit of those who don’t have access to welding equipment, and wouldn’t trust a local garage to do it !