Adjusting guitar intonation

[Maccy]

Have an annoying top E string, anything past the 12th fret and it drops in tuning by just under a semitone.
Have tried adjusting the screw at the bottom but this isn't helping at all, anything past 12th fret still remains flat.
The strings are fairly old and I've ordered a new set of Elixirs but is there anything else I'm missing? The guitar is in tune on open strings and there's no problems with any other strings. It's an American Strat.

 

[Stev]

http://www.guitarattack.com/repair/intonation.htm

though Im sure you know this. if they strings are old maybe they are just overstretched?

new ones should do the job

 

[Maccy]

Think I may have cracked it, the screw just needed loosening a lot more than I thought and it's done the job until I get new strings