On the initial forms I had family friends give evidence in this section, however the claim was rejected and I had to request a reconsideration. At that point I approached all the instructors from J's out-of-school/home activities (karate, swimming and football) and got them to write reports on how much additional support he needs while in their care, plus how his behaviour differs from the rest of the group. Obviously I'd primed them first with the type of thing the DWP are looking for, and got them to send me a draft first to approve before sending it off.
In addition, I compiled more notes on my own involvement in getting him to these activities - the encouragement he needs to go in the first place, reminders of appropriate behaviour, rewards for achieving etc, plus the time it takes to get changed/dried etc.
The nursery teacher doesn't have to think he's autistic as it doesn't matter what the dx is but the additional care that's required. If he needs more attention from her than other children do then her input could be useful. Remember too that you aren't restricted to only one statement of evidence, you can provide as many as you like from different sources. They might not read them all but you can refer to them if you need to request a reconsideration at a later date.
I also had a report from the Health Visitor, though in the first instance it was ignored (she'd actually said the words "J needs significantly more supervision than other children of his age"), but at reconsideration I got her to help me compile a list of age-appropriate behaviours and how J doesn't match up to them, which I think seemed to help.
Don't worry about missing anything at this stage. You'll get chance enough to fill in the gaps at reconsideration if you need to. Best not to fire all your bullets just yet, IMO. At this stage your claim will be assessed by civil service pen-pushers with no medical experience and you may not get an award simply because they lack the insight into DS's condition. Just keep plugging away and you'll get there in the end.