Hi
We're in a very similar situation - waiting for assessment/dx for ds (probable asd, possible HFA). We've just started an ABA programme - the VB (verbal behaviour) method. We have set it up ourselves with a consultant, supervisor and 3 tutors. The first step is to find a consultant, then recruit tutors yourselves. We used gumtree and aba yahoo group to find tutors (we interviewed a lot!). It has taken us about 3 months to get it all set up.
As justa said, it is a full-time thing - for it to be effective you need at least 20 hour per week, so I have accepted my life is going to be pretty much devoted to it for the forseeable future. Ds has two sessions every week day, 3-hours each.
No you don't need a dx - anyone can do it.
It might be worth contacting BIBIC too, they do assessments and though don't diagnose, they offer some help with strategies, advice and suggestions of therapies. But there is a very long waiting time - we're having to wait till March 2011 for our assessment!
I'm in SE London too
Whereabouts are you (without giving too much personal info away?). How old is your ds?