I have arranged for my NL1520 to be picked up for repair, hopefully they'll be able to do something with it. The touch screen issues are intermittent but frequent enough to really annoy me every day! The Lumia 1520 I have is a recent one and already came with Lumia Cyan preinstalled.
So there's no way of rolling back to Windows Phone 8 to test touch screen issues. I'll keep you all updated with the outcome.
I am pretty confident that this issue is hardware related as discussed in other threads in this forum. It needs to be sorted or I'll end up getting rid of the Lumia 1520, I'll be sad to say.