Nationally, approx. 30-40% of appeals are successful. The last figures I saw can be found here:
http://www.dfes.gov.uk/rsgateway/DB/SFR ... 1_2005.pdf
The most striking difference seems to be not between grammar and other types of school, but between local authority and non-local authority schools.
The success rate for foundation and VA schools tends to be around 10% lower than usual (i.e. approx. 20-30%). I have written elsewhere of my reservations about the conduct of appeals in some
non-LEA schools. See, for example, section A5:
http://www.elevenplusexams.co.uk/11plus ... nswers.php
The ombudsman, in his special report on appeals, wrote: "We have observed differences in performance between LEAs and other admission authorities ..... we found faults proportionately in virtually twice as many cases involving other admission authorities as compared with cases involving LEAs (39% and 20% respectively)."