OK. I get it.
So you sent the initial request and they only have 6 weeks from when it reaches them to decide if they will assess.
It sounds like something had delayed the initial response to your request. They want you to send them any information which might help them decide if your DS needs a Statutory Assessment.
So, although you don't have to send anything, it's your chance to convince them. I would do this:
-Make photocopies of any letters you received from a professional about your DS (not appointment letters, just the clinic letters that you got after an appointment)
-copies of any IEPs or any written statements of your DS'S needs.
-copies of his diagnosis letter
-copies of SALT/Ed Psych/OT letters.
Send it all with a covering letter which summarises his needs: dx of ASD. Language approximately of a 15 month old. Very slow progress. Unlikely to catch up with his peers in the short/medium term, etc.
One thing you need to realise is that bad=good in statutory assessment land. You need to be as bleak as you can bear to be.
So, if your DS had one word last week and gained another today, in RL you'd say 'wow, DS has doubled his number of words this week.' In SA land you say 'although DS has gained one new word, same age peers have developed thousands of words and can converse with children and adults freely.'