Apologies - he was tested with WPPSI. The GLC is the General Language Composite. The way the psych explained it to me was that his Input - what he is learning is very high (152+) but his ability to demonstrate his learning was average. So like when you know the word you want to use but you just cant think of it. She said that if James was tested verbally only he would do very well at school but it is his demonstrated work that is suffering, particularly reading.