if you know how to fit a door,have a look at the top of the door to see if there a trim or big silicone line,if so remove some and see how big the gap is,if you have about 10mm,take door out and re-fit after you have packed door up as much as you can,if not take frame out,cut about 10mm off head and pack up and re-fit.Make sure your sash has enough clearence at the top to allow you to do this.hope this helps
shaun www.sdwupvcrepair.co.uk, March 2009