leocg, thanks you so much for taking your time to look into solutions.
I hadn't made the effort to try other browsers, as it mysteriously coincided with me updating opera. It had been working fine, and then as soon as I restarted opera after the update, it stopped.
I just downloaded Chrome and tried that, as you pointed out it has the same issue, so it appears it isn't specific to the Opera update.
My apologies for the rookie error in my troubleshooting, sorry for the hassle!