Thanks everyone. TClanger - will get an email off to you soon-ish, if that's ok?
TotalChaos: thanks, that's really helpful re: SALT input. We have aleady started some feature/function/class stuff with her beloved photos and flashcards (so, instead of asking her to give me a picture of dd2, she gets asked, which on eis your sister? or, instead of asking her to give me a picture of a bird, it is "which one has feathers" or "which one flies in the sky". Slow work, but we're getting htere inch by inch)
cyber: dd1 has a generic past too - usually 'done' or 'had', so typically she would say
i had raisins
i've done painting
I've had Something special, etc. Where she does use a different (ie more accurate) verb, it is normally something she has learnt as a whole chunk - as soon as she is generating language herself it is back to generic past. When she doesn't want to do something, it is either "finished" or "tomorrow", so ask her if she wants an apple, eg, and she will say "apple tomorrow"
we were recommended to model appropriate language for dd1 too. We completely cut out questions for her (which is why she is not so good at handling them, I think, but it did serve a good purpose), and only presented her with what she should be saying. So, no asking "do you want a drink" just saying "mummy, I want a drink", and if she comes up to us and wants something, she has to ask properly. So, "mummy read it this one" is no longer good enough (she is brilliant at vocal imitation) and we supply "read it to me please" (or similar) and she doesn't get it unless she copies.
Sadly, where we talk to dd2 as well, dd1 is picking up loads of references to her - a recent favourite is "give it back to dd1" - something which we say to dd2 a lot (she is 22 months, and constantly taking things off dd1) - trying to remember to model "gve it back to me" is difficult, especially since dd2 picked that up all too quickly, and so dd1 still lost out . So pronoun confusion is here to stay for a little while longer, methinks.