Thanks
@Matt_01 and
@Cruiser Elite for your advice. I called SQ this morning and have now confirmed and ticketed Mrs Ken1 in F Saver for SYD-LHR on SQ242/306 on the original date we had booked the J flights. There was still an F Saver seat available for the day before so I have now booked and ticketed to travel on the same flights a day earlier than Mrs Ken1. I have put in waitlist requests for myself for the date of her flight and for Mrs Ken1 for the date of my flight so if one of the waitlists are confirmed we can travel together, but if not at least we are still getting to London. This will be our first time flying F so we're looking forward to it.
I can't click "Manage" for either of our bookings to try to select seats - it is greyed out. Also I had to go in and "Add frequent flyer number" for both of our bookings and even after I added our KrisFlyer numbers it still shows "Add frequent flyer number" when I go back into the bookings. On the original J booking our KrisFlyer numbers were on the bookings automatically. I'll see if the system updates overnight and I can go into "Manage" tomorrow.