Update:
Received my 1520 back from repair yesterday. It took a week from courier pickup to getting the phone back so fair play to Nokia for speedy service. FYI my phone had 8.1 on it and they repaired it without issue, although they did flash it back to WP8.
So it looks like the screen has been replaced as the small light leakage patch has now gone. Sadly the swipe/tap problem is still there, so I will get the phone setup properly again and will uninstall the Touch app and see what effect it has. There seems to be plenty of anecdotal evidence now that doing this fixes the swipe/tap issue, but I'll test it myself and post an update again.