Happy birthday, CV! And many happy returns.
MI, hope the phone call to Norfolk social services is fruitful. And sorry to hear that dp was less than supportive at the weekend. What is he expecting you to do, exactly? Magically add another 12 hours into the day so you can zoom up and down to Norfolk at the drop of a hat and get everything else done and still get enough sleep? Hmmm.
Thank you, one and all, for your sympathy and wise words. I think that restricting wifi access for a week or so did make some kind of useful impression, as dd's room is tidy and there is NOTHING on the floor!! And her schoolwork has been filed and the desk is actually looking as though she can work on it. Her wifi access is now only restricted between midnight and 7am, which she agrees is reasonable. All the rest of the time, it will be up to her whether to fritter away her time on Snapchat or Whatsapp or whatever it is that she is doing. Being ultra-calm about her report did seem to pay dividends last night, as she was also calm about it and got on with some revision yesterday evening. Result! However, dh was being an utter bad-tempered pain, and huffed and puffed about her still having the wifi on after 11pm because he was adamant that it was unnecessary. He is mainly vv cross because when she got really furious the other day, she said we couldn't complain about her doing things that he does e.g. not getting dressed until 3pm, spending hours faffing on the computer, leaving dirty plates and cups all over the kitchen and not helping around the house. I said it was a fair point. He has also now taken up moaning at me for staring at my phone all the time. 
Dh doesn't have PR for dd, so doesn't exist as far as the school is concerned, i.e. he can't give consent for any trips or medical stuff and all stuff from the school is only addressed to me. He has never been to any of her parents' evenings and I'm not sure the school would talk to him if he turned up anyway. More importantly, perhaps, he wouldn't have a clue about anything they said and would inevitably get the wrong end of the stick and she would be completely enraged if he went anywhere near her school in the capacity of a parent. (There is also the small matter that now that ds is at middle school, dh has decided that he is coming with me to parents' evenings, thus highlighting the great divide between his attitude to dd and his attitude to ds. And yes, I have spent a decade pointing stuff like this out.)
Anyway, I have decided that I must try to model grace under pressure (arf) as the shouting strategy no longer works and yesterday's calm persistence (whilst frothing and seething under the surface) paid off, after a fashion. Must remember not to lose temper at dh instead of at dd. That won't end well either.