Rights whilst waiting for assesment?

I think that all children should be having their needs met in an educational setting - autism diagnosis or not. Is there something in particular that your daughter is struggling with? Are you in touch with the Senco at your daughters school?

Well if her school are the ones that recomended the assessment it is likely they identified certain difficulties she has been having at school already, in which case she is likely already registered as having SEN with the school but you can also call the school and just ask them directly.

My kid was already on the SEN and has a EHCP but official diagnosis is only just starting to be talked about by the school, but that is for longer term supprt, he already gets additional support simply by being on the SEN and having an EHCP, so I don't see why the would say no to you asking for additional support for your daughter. Incidentall if you haven't heard that term before from the school you can ask them to begin the process of getting an EHCP in place for her.

Without sounding ignorant, how  do I know if she is in the SEN register? 

Is she on the SEN register? If so I believe she may well have the right to ask for some reasonable adhjustments based on observed needs even in leu of an official diagnosis, but you may need to ask her SENco officer.