Have not reassembled the bottom bracket yet but I have cleaned the threads using some of this forum’s tips and now the cover goes on really easy. I can screw it in all the way just using my fingers. It took swearing and sweat to move it before. I will endeavour to keep it as clean as possible in the future. I made a tool like ranyjawa’s and it worked a treat to loosen it when it was dirty. At least now I know what I am dealing with here. I will get back to you when the job is done. I am really grateful for the advice.