Yes, thread for the bottom nut was a left hand, and top nut had a right hand, i.e. as you said Matthew. Both undid with a sharp tug on the socket bar with an extension fitted for extra pull. New cutter heads now fitted and working very well.
Thanks for all the help
I have just bought some lovely new Whitehill cutting heads for my Multico tenoner and am now having trouble getting the old heads off. Is the nut holding the lower head a left hand thread, and a right hand thread for the top head? Or is it vice versa??