Google ads sometimes escaping my container...

Hi all,

Yesterday I added some google ads to my site (AdSense) and added them into the header and the sidebar. Being an infrequent CSS user I was very surprised and pleased that they appeared where I wanted them the first time...... however, since then I've noticed that they are not quite behaving as I wanted them to.

The sidebar one is fine and behaves as expected but in the header I've put 2 ads side by side. A lot of the time this is working fine but sometimes the right hand set of ads is jumping to below the left set....... hopefully you will see what I mean when you look at it.

The site is http://gbwildlife.co.uk/ and the css code I've used is:

#headerAds {
margin-bottom: 10px;
width: 100%;
height: 120px;

}

#headerLeaderboard
{
float: left;
/* background-color: red; */
margin-left: 15px;
margin-right: 18px;
margin-top: 10px;
padding-bottom: 10px;
margin-bottom: 10px;
width: 730px;
height: 90px;
}

#headerLinkunit
{
float: left;
/* background-color: red; */
margin-top: 10px;
padding-bottom: 10px;
margin-bottom: 10px;
width: 200px;
height: 90px;
}

I added the #headerAds div to try and contain them as the first time I tried and the right hand add jumped underneath it also sat on top of the content and made a mess - at least now when it jumps it just pushes everything down the page!

Can't quite figure out what I've done wrong here as the site width is 995px and the 2 ad divs are only 963px in total??

Thanks for any help

Andrew