yellow/orange circles on screen?

I've started to get 2 "circles" (seem to have a feathered edge to them) towards the bottom of my screen once I've used my phone for 10+ minutes. A reboot gets rid of them, but they are quite distracting and annoying.
One is around 1.5cm in size, the other is about half the size, they appear whenever the screen is on (including lock screen) except when something black is there (though it's probably that I just cannot see them!)

is this a hardware problem?
really don't want to have to re-set my phone if I don't need to.

If it's a hardware problem, does anyone know how I can get it fixed or replaced by Google UK?

Take a screenshot and see if they show up on that. If they do its probably software, if not it might be hardware.

nothing there on a screenshot (when viewed elsewhere)

Photo of screen: [Only registered and activated users can see links. Click Here To Register...]
Yellow bits circled as the photo doesn't do it justice: [Only registered and activated users can see links. Click Here To Register...]

Edit: leaving it on a white screen and charging at the moment, I've just noticed a much bigger one appearing around 2x1 cm half way up on the right hand side :(

the new bigger mark:
[Only registered and activated users can see links. Click Here To Register...]
circled so it can be seen: [Only registered and activated users can see links. Click Here To Register...]