If you are interested in early intervention you need to contact RGO (on mumsnet) or you can PM me and i'll give you her details, as far as I know she is the only person in the UK trained to deliver the Early Start Denver Model and it is based mostly on what you can do at home. She is brilliant and I cannot recommend her highly enough.
We got a diagnosis of ASD at 2.9 yrs and told our daughter would most likely not get to go to a mainstream school but it would depend on a few factors.
Our dd developed normally up to around 14 months, only sign of autism was her lack of eye contact when we fed her when she was a baby. However, at 14 months she changed dramatically. Complete 180, would describe really as severe autism, no communication, eye contact, no interaction with anyone, would just sit in place and stare or pick up rocks and throw them over her shoulder for hours if you let her.
We knew getting a diagnosis was a long process so we just started doing stuff, reading, got her a few sessions with a private SALT, private OT who did lots of sensory integration therapy and this made a HUGE difference. I started talking to RGO who gave us lots of tips and advice before we had our diagnosis and its been 9 months since DD has had her diagnosis and we've been working with RGO and doing heaps at home ourselves and with volunteer SALT students. DD's diagnosis now is unmistakeably HFA, she talks, enjoys mainstream nursery and will most definitely be going to a mainstream school.
A lot can change when they are young. So what i'm trying to say in a very long-winded way (sorry!) is that regardless of how severe they might seem early on a lot can change and anything you do will be very helpful to your child even if you haven't had a diagnosis. Don't worry about how severe you think it is early on because most times that can and will change. Our paed said that only in a handful of cases in all her career did she see any kids that didn't move up on the spectrum.
A few books i've read and highly recommend are:
Overcoming Autism by Lynne Koegel: www.amazon.co.uk/Overcoming-Autism-Finding-Strategies-Transform/dp/0143034685/ref=sr_1_fkmr1_1?ie=UTF8&qid=1307282233&sr=8-1-fkmr1
Raindrops on Roman: Overcoming Autism by Elizabeth Scott: www.amazon.co.uk/Raindrops-Roman-Overcoming-Autism-Message/dp/1934759244/ref=sr_1_1?ie=UTF8&qid=1307282271&sr=1-1
This last book is awesome because she tells you exactly what she did at home and other than SALT once a week and OT she did everything herself from very early on and the book has heaps of ideas of things to do to help.
Until you have a diagnosis it will not hurt to start doing positive things to help your child. Please feel free to PM me!