One thing I did notice was that the class declaration
clearboth (this is as it appears when you view the source of your page) and only for sections of the header portion of the page.
does not match the declaration in the stylesheet.css which is
.clearBoth
<div class="header01">
<a href="http://www.ukbikegear.com/"><img class="logopoz01" src="includes/templates/jsweb/images/design/logo.jpg" border="0" alt="" /></a>
</div>
<br class="clearboth" />
<div>
<div id="login_header" class="back"><form name="account_box" action="https://www.ukbikegear.com/index.php?main_page=login&action=process" method="post">
<table border="0" cellspacing="0" cellpadding="0">
<tr>
<td align="center" valign="middle" class="text_top">Login </td>
<td align="center" valign="middle"> <input type="text" name="email_address" size="15" value="e-mail" onfocus="if (this.value == 'e-mail') this.value = '';" onblur="if(this.value=='')this.value='e-mail';" /> </td>
<td align="center" valign="middle" class="text_top"> </td>
<td align="center" valign="middle"> <input type="password" name="password" size="15" value="password" onfocus="if (this.value == 'password') this.value = '';" onblur="if(this.value=='')this.value='password';" /> </td>
<td align="center" valign="middle"> <input type="image" src="includes/templates/jsweb/images/design/login.jpg" alt="Login" title=" Login " /></td>
</tr>
</table></form>
</div>
<div id="cart_header" class="back scpoz"><div><a href="http://www.ukbikegear.com/index.php?main_page=shopping_cart ">Shopping Cart</a>: Item(s) <font class="font_red"><B>0</B></font> | Total: <font class="font_red"><B>£0.00</B></font></div></div>
<br class="clearboth" />
</div>
<br class="clearboth" />
<br class="clearboth" />
I'd suggest going through tpl_header.php and correct the those clearboth calls and that should clear up the problem.