I’ve read all the comments and reread my licence and think I now have good understanding of the matter. I’ve written a guide, Am I Mobile or Portable?: Am I Mobile (/M) or Portable (/P)? Amateur radio callsign suffixes
I’m not sure it is “finished” but I’ve wasted far too much time on this already which should probably have been spent doing something more technical or useful.
Thanks to everyone for their comments, particularly those quoted.