Thanks for your response.
I've had that happen many times before (the login timing out meaning I can't save the changes) and at those times, I've logged back in on a seperate tab and then saved changes. I'm not sure what the error was on this occasion but I'm now able to edit the page as normal anyway.
Thanks so much for for that tip! I've followed what you said and now finally have sensible URL's. I should've asked this question a long time ago!