thanks again for all the support and advice.
for those who asked, i'll fill you in on how it's going. for those who didn't, this will be long and boring, so get the thread hidden.
i don't want to jinx things, and i'm sure we're not out of the woods yet, but we had a really good night.
dd2 went down at 7pm. awake in cot. no pupd. she woke at 10.15, 2.50 - went down probably asleep after a good long bf each time. i didn't try to resettle her first, just bfed. at 6am dd1 came in and started mithering that the cat was bothering her, which may or may not have caused dd2 to wake up. which she did. bf and up for the day.
the naps were pretty gruelling yesterday though, with dh doing pupd for 40mins for one, and dd2 fighting sleep hard. i also spent some time doing pupd to extend one of the naps, which worked - she went on to sleep for another 2hours. i did this because on waking she was crying, with her eyes tight shut, trying for all the world to suck her fingers, and looking desperately like a girl who wanted more sleep.
I am working on the theory that she has been chronically overtired, and trying to get her to nap frequently and for as long as possible. her tired signs seem almost non-existent, so it's been really hard to hit the sweet spot between not tired and overtired, especially with dd1 being around. we've not gone for a routine yet, as i'm finding just trying to get her to sleep when tired is hard enough.
after having got the longer stretches at night, her bfs felt longer, with stronger sucking and more gulping. i think that previously the frequent waking and then snacking for 20 seconds or so before unlatching might have added to the overall difficulties in getting any stretches of sleep longer than an hour.
anyhow, i'm not expecting it all to be plain sailing from here on, but i feel an awful lot better after getting about 6 hours sleep last night (albeit in 1, 3, and 2 hour chunks). no more than a handful of 20 minute snatches of sleep over a week , was really too little for me to function. the four day long headache has gone now too.
am off to help dh with the dds.