I think I may have fixed it, maybe not in a preferred way (I always seem to come up with an answer as soon as I post here....?!)
I combined two cards (date and time) and set to the date-obs card in the header before passing to super().__init__ - should I be doing something different?
Also, just out of interest, is there a mechanism to access fits file extensions in the Map class? The CoMP data comes in a fits file with 4 ImageHDUs (Intensity, Doppler, etc). At the moment, I am using a separate function to split off the data product I want before passing in the data and header to GenericMap. Is it possible for me to pass in the extension name to GenericMap?
For example, with the astropy fits reading tools, I can say HDUL['Intensity'].data and get the intensity ImageHDU.
However, lets say I just wanted to pass the file path to GenericMap and the name of extension I required. Can this be done?