Well we haven't cracked it after all. Last night BabyM got up out of bed every time she woke up. Eventually, at about 5am I put her into her cot and she finally slept til about 7am.
She's exhausted today, but when we put her in her cot for a day sleep, she climbed out! I feel like I just can't win. Hubby has even suggested getting a lock for her bedroom door, and I'm almost at a stage where I will agree.
We've decided to just get a round handle for now, which she won't be able to open for a while, and I'll put a mattress on the floor so if she does climb out of her cot, she won't be able to get out of her room, but she'll have a comfortable place to drop off on.
I feel like such an evil parent, effectively locking my child in her bedroom til she conks out on the floor from exhaustion, but we have tried everything else we can think of.