TwentyTwelve IE9 issue

Hello

I’m currently in the process of designing my blog, I’ve made a child theme from the TwentyTwelve default theme and have almost customized it to the way I want it to look. However, I’ve suddenly started having the problem that the top menu/navigation bar, it shows up fine in Google Chrome and Firefox, however in Internet Explorer 9 which I’m currently using it puts each link on a new line so instead of:

Home | About | Contact

it’s got:

Home
About
Contact

which is really annoying and obviously doesn’t look good! Anyone got any ideas how to fix this??

Many thanks!