I note you used WD40. In my experience that is a water dispersant and not all good for freeing seized parts. I find Plus-Gas that has been mentioned above so much better and is actually made for the job. You can get it from any good motor parts dealers and maybe even Halfords.
So I would echo Bob's comments and say, start with a good soaking of Plus-Gas.
in the past I have had excellent results on siezed brake slave cylinders by immersing in vinegar which is a weak acid Acitic acid) that eats the rust away, build a coffer dam of plasercine and fill with vineger leave for 48 hours and "gently " tap apart.
Then make sure you wash all the vinegar away and clean the metal up.