If gap is realy big it sounds like the frame needs adjusting,take out the screws in head of frame,push frame out slightly in center of the head and drill and screw it securely,make sure you cut silicone inside and out or it wont move,if gap is small raplace rubber gaskets around doors and frame with a thicker bubble seal and should be ok.hope this helps
shaun www.sdwupvcrepair.co.uk, August 2008