OK the plot thickens
Ive sent the phone back to Nokia, but they say the display is working normally.
I went to a mobile phone shop and had a look at the 925 and the 1020 they had.
I put them both on the light theme with crimson tiles and they both had the same problem - i.e. when scrolling on the start screen quickly the colour of the edges of the tile 'bleed' onto the white background, presumably because the pixels cant react fast enough to the colour change - almost like the tiles are trying to join together.
To me it looks quite bad, and not a theme configuration I would want to show the phone off with.
My 625 and my 520 are fine, but then again they have a lower density screen based on a different technology (as far as Im aware).
This really suprises me, and it suprises me that I cant find any other mention of this issue on google. Perhaps the light theme isnt very popular for general use, and no-one has seen the issue. It's made me a little dissapointed in the phone, and Im going to have a proper look at the 1520 in the flesh befor I buy it, and if it has the same problem I might wait to have a look at the 1320.
Does anyone have any experience of this issue. Ultimately I could be completely mad and ready for the loony farm - I havent totally ruled that out as a possibility.