Thanks for your advice - much appreciated. I booked via the JAL website, and I am a JMB member so would assume I can make all substantive changes via the website, but this doesn't appear to be the case. Somebody else has suggested contacting JAL Customer Service in Japan but whenever I call the English version it's an hour long wait with no pickup. Would you suggest calling the Japan version?
The Osaka flight was made as part of the LHR-HND flight so based on what you've said, that should be updated too. The only issue remaining is when I will be notified of this connection so that I can make changes to my itinerary ie hotel stay in Osaka on return to catch the domestic flight in the morning or overnight stay in Tokyo.