Thanks guys.
Melssy, thats exactly like DD.
I'm trying to up her formula a bit. She drinks 120-150ml and then I'm offering her another 30ml before she gets back to bed about an hour later. She only takes about another 10ml and then spits it out.

By afternoon she's hungry, but she just 'chomps' on the bottle and gets cranky.
I'm off to weigh her this afternoon to see if she's put on some more weight.
I'll see how she goes today, I may ring the surgeon this afternoon. He's usually fantastic, I've rung him a few times since being home and he's always rung me straight back and is a massive help.