Thanks to those who told me that the LH Gnutti cotterless crank fixing bolt has a left hand thread -- I would never have guessed and would otherwise have continued to ply it with penetrating oil and tried to "unscrew" it. It unscrewed quite easily once I turned it the right way. It would seem like this threading was an unnecessary precaution on the part of Gnutti -- I have never had a problem with a cotterless crank retaining bolt unscrewing itself and have not heard of anyone who has.
Hugh Thornton
Nantwich UK