Had a great plan from my health visitor.
I did a sleepy feed at 12 midnight (was meant to be hungry baby formula but I was full of milk so give him that first, then he took 2 oz hungry baby formula after!).
Then when he woke at 3, I tried to settle him with a dummy, but it only worked for 15/20 minutes so I have some boiled and cooled water in a bottle. He drank an ounce then settled back to sleep. No trying to play and no vomiting.
Next wake was 6:30am for a feed and to start the day. He was hungrier in the day and so took more milk, hopefully reversing the nocturnal habits!
I was quite encouraged by this and will let you know how it goes. It certainly seems to be the right thing because the night feed was likely to be a habit rather than a physical need as it was the only feed he vomited back up which suggests he didn't need it.
Lindsey - I was told once that with a young baby you should let them sleep too long in the daytime - to make sure they have a feed every 3 hours IN THE DAY.
This means they will learn to sleep longer at night when the time comes later, but you can't expect that immediately. Best thing you can do is make the day the day (and not the night) with feeds. You don't have to forever, just in the beginning.