Just a quick update, as didn't get to sit down til 9:15 and hoping to go to bed at 10!
She got up once in the night and came in to me, and told her that it was the middle of the night and to go back to bed or the gate would go back on, and she went straight up.
Tonight I explained the routine that we were going to follow, and each time she started messing around (splashing too much in the bath, shouting in ds' face etc) I reminded her that it would mean no boo-boo before bed, and she would apologise and be better for a few seconds until she forgot again . Anyway, we left ds to whine and went up for boo-boo and bed for dd. I reminded her of what dh had explained yesterday about us using the gate until she had proved to us that she could be trusted to stay in bed, but when I said that the gate would go on she tried to dash off the bed past me to get downstairs first. I beat her and put the gate on, and explained through her screaming what was happening, and then went back to put ds to bed.
Cue half an hour of screaming and tantrums from dd (mummy, help, I don't want this, I want daddy, scuse me etc), until I'd got ds settled (I popped back to her once during this), and then I went up and had a talk with her, and opened the gate with her understanding the consequences if she came down. We had cuddles and she stayed in bed .
And ds is asleep in the Amby tonight. He had a good nap in there this afternoon (am following some advice from Aitch) so thought I'd give it a go.