We do not have a Grammar school system gernerally in Hertfordshire. In South West Herts there is a Selective Consortium of 6 schools:
Watford Grammar Boys
Watford Grammar Girls
These schools have a policy of selection (up to 35%) of their intake based upon academic abilitity. The tests comprise Maths and Verbal reasoning.
The prospectuses state that if you think that your child will be disadvantaged in any way by reason of disability then you should provide evidence of this for concessions.
click on admissions criteria 2006 for the prospectus. Relevent bit about which tests are applied is P4, special considerations is on P5
Concession for dyslexia is extra time up to 25%, and yes it does get awarded.
Hope this helps.