My daughter was diagnosed last year at the age of 10 with very severe dyslexia. I had been going on at the school about her lack of progress and all she had was extra reading. We paid privately to have a psycholgical assessment done (�400) and it was the best money we ever spent. Armed with the report I tackled the school and was given loads of info. Your neice may be on School Action or School action plus (depending on her needs). If she is, she will have plan of how her needs are met which her parents should be involved in.
As she is in year 6, I feel that school have kind of 'written her off' and I feel she will be more appropriately supported at secondary school.
Have a look at the SEN code of practice
http://www.teachernet.gov.uk/_doc/3724/SENCode OfPractice.pdf
or get her parents to get in touch with your local PARENT PARTNERSHIP
http://www.parentpartnership.org.uk/
Best of luck