Welcome to the Community, @timcycles.
I've just given this a check myself, and it's letting me get as far as editing the address, and sends a one time PIN to allow the change.
I'd recommend giving this a try from a different browser / device if possible, and could be worth logging in via a private browsing window to rule out the possibility of a cookie-based issue.
If you continue to struggle from there, I'd recommend giving our team a call, and they can help you change this, and check how your online account is set up too.
Peter
