Do you mean you made a request for statutory assessment, lowexpectations? Did you make it, or did the school?
You can appeal against a rejection to assess, and if you have missed the window for this, you can apply yourself (if the school made the last application) and vice versa.
Look at the IPSEA website for model letters. Parent Partnerships (although I have my doubts about how impartial they are) usually have some good information leaflets, including a guide to timescales for the whole assessment/statementing process (how long you have to submit info, appeal etc). Look on your LEA website - usually where PP services are found.
To be honest, as supportive as the SENCO is, if she is inexperienced and unable to provide the support and information you require, she is not doing her job properly. She needs to be proactive in getting advice from the LEA or outside agencies. If she is unable to do that, I would request (in writing and verbally, to the Headteacher) that the school/LEA seeks outside advice immediately. My son's school, for example, seek advice on autism from a local special school.
Obviously if she is supportive it'd be nice to keep her on board, but at the same time she has to step up and do her job!