You need to talk to your exam centre and they will talk you through the process. Not all exam centres can/will do access arrangements. Commercial centres are more likely to be able to do access arrangements for private candidates. This might help he-exams.wikia.org/wiki/Access_Arrangements
Also ask on your local home ed group. People there may be able to tell you about local centres where their children have been able to get access arrangements. It has to be sorted out quite far in advance, and you may need to travel some distance for a centre.