Tried cleaning it off. It's under the top layer and I can move it with my fingers around inside, similar to getting water between a digitiser and glass on an old style lcd screen.
Touch panel still works, can't see it when illuminated, bizarely it has exact straight lines that are in from the edge of the lcd - with block keyboard up approx one letter in from the edge either side fully top to bottom. Not been dropped, wet or abused but has been in pockets with no case / screen protector.
Any ideas? Anyone else got it? No photo shows it up :-( Time to call orange care methinks. Had paid for an unlock for it too :-(