Two links for you:
RNIB | click here
Bobby | click here
The RNIB have an amount of informaiton on their website and will give you some guidelines. Bobby is, as their site says, comprehensive web accessibility software tool designed to help expose and repair barriers to accessibility and encourage compliance with existing accessibility guidelines.
A well designed website should be accessible for anyone with a disability or visual impariment. The RNIB is very much against the development of 'sister' sites where you have one for 'normal' people and one for 'disabled' people.
(NB: My comments are not intended to cause offence to anyone reading. Words have been chosen because of common understanding.)