You can only get into the ground floor of the Weston Library if not on a tour (or possessor of a readers' card). This is where the free exhibitions are as well as the tour office. There can be extra on the day tours subject to availability.
Thank you for the tip. I just checked and there are at least two free events that I'm reasonably interested in, especially considering that it's my only opportunity to get inside. They are in the Treasury and in Blackwell Hall.
Both events say "Free, no ticket required". Does that mean that there are no limits on attendees, or might they still fill up?
Admittedly a year and a half ago, but when I went there were some tickets saved up for sales onsite. I was fortunate to get the last one. So it might pay to go early in the day and see if any are still available.
Good suggestion, thanks. There are (fully booked) tours scheduled both days we're there. Our hotel is a 15 minute walk away so we can check easily enough.