I've got the same bike and setup. The metal washer you can see should lift out, and there should be a spring underneath. Once there, you'll be able to see the expander.
Just as you said, thanks.
@richardaldred started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
I've got the same bike and setup. The metal washer you can see should lift out, and there should be a spring underneath. Once there, you'll be able to see the expander.