They were fine, I just said when they came round in the morning to do my obs that I would be going home that afternoon so if there was anything they needed to get done before I went home that they'd best make a start on it asap
They sent a registrar round to see me who explained why they'd like me to stay in another night but I just thanked her for taking the time to see me and that after listening to her advice I had decided that I would still be going home. They did ask me to sign a discharge against medical advice form(which I did) but you don't have to sign anything, they can't keep you there against your will.
I find as long as your polite but firm(i.e don't 'ask' to go home just tell them that's what your doing) that they're fine.
P.S I would have gone home after 17 hours but they wanted me to stay for my clexane injection as they said they had to be so many hours apart, when I got home my community midwife told me it was rubbish and that she certainly wasn't coming round my house at 7pm for the next 4 days to give me the rest of the course