Try 0.7-0.8 mm gap. This is for an MY97 WRX, although the UK equivalent is 1mm (only heresay).
Also make sure the lead caps are firmly pushed on with a click.

 

I have a 93 wrx and have just replaced the plugs with NGK platinum does anyone know the recomended gap for these plugs?
as i am suffering from a misfire at around 5000 revs under heavy acceleration and think it could be due to the plug gap size.
thanks
Richard