ooh, blimey, now you're asking.
I'd have to look back at her red book to be absolutely sure of what I went to the hv about, but off the top of my head:
her babbling was there, but not very varied. no range of sounds, and there didn't seem to be any communicative attempts behind the babbling.
she was on the late side of normal for all milestones (rolling, sitting, grabbing/holding things, pincer grip etc) and just didn't seem to be quite right with any of these things. she did hit the milestones, but there was always a 'hmm, she is doing that, but ...' about them all.
she didn't really reach for things, or grasp them - cot toys/buggy toys/mobiles etc, no batting at them, no realisation really that she could even attempt to do this.
she slept a LOT. really, really a lot. as a baby, she would easily have a morning nap of an hour+, an afternoon nap of up to 3 hours, and then sleep 7-7 as well (I mean as an older baby - 8-12 months). it seemed as though she found everything too much, and was retreating into sleep.
as she got a bit older (again, the 8-12+ months range), she didn't play with anything, at all. no interest, no inverstigation. if I put rattle in her hand, for eg, she would hold ituntil her grip failed, then it dropped and she would not look for it. same with any toy. and then, if something did take her interest, she would play with it repeatedly, for far longer than her attention should hold. I remeber sitting with the hv for about 40 minutes once, when she was just over 12 months. she picked up and dropped a single brick for the whole time. no interest in us, what we were doing or saying. no need for our attention, just total focus on this one brick (a whole tub was available, which she ignored).
I know that a lot of these things are not that far off NT, at itmes, but it was the frequency and intensity of them that meant that something was wrong. the whole picture was more than that, iyswim?