User login block does not appear in Internet Explorer

A weird problem has started with my installation. Not sure when it started, as I mostly use Firefox, but the user login block is not showing in any version of IE.

It appears in every other browser I've tried (Firefox, Opera, Chrome, Safari), but in IE the block just doesn't appear in the page source.

Here is the source as viewed in Safari:

<li class="leaf last"><a href="/contact" title="Contact Plan UK using a form">Contact</a></li>
</ul></div>
</div>
<div id="block-user-0" class="clear-block block block-user">

  <h2>User login</h2>

  <div class="content"><form action="/home?destination=node%2F120"  accept-charset="UTF-8" method="post" id="user-login-form">
<div><div class="form-item" id="edit-name-wrapper">
 <label for="edit-name">Username: <span class="form-required" title="This field is required.">*</span></label>
 <input type="text" maxlength="60" name="name" id="edit-name" size="15" value="" tabindex="1" class="form-text required" />
</div>
<div class="form-item" id="edit-pass-wrapper">
 <label for="edit-pass">Password: <span class="form-required" title="This field is required.">*</span></label>
 <input type="password" name="pass" id="edit-pass"  maxlength="60"  size="15"  tabindex="2" class="form-text required" />
</div>
<input type="submit" name="op" id="edit-submit" value="Log in"  tabindex="3" class="form-submit" />
<div class="item-list"><ul><li class="first"><a href="/user/register" title="Create a new user account.">Create new account</a></li>
<li class="last"><a href="/user/password" title="Request new password via e-mail.">Request new password</a></li>
</ul></div><input type="hidden" name="form_build_id" id="form-ae3909eda750f31bfc060289345ddf65" value="form-ae3909eda750f31bfc060289345ddf65"  />
<input type="hidden" name="form_id" id="edit-user-login-block" value="user_login_block"  />

</div></form>
</div>
</div>
        </div>
      
      <div id="center"><div id="squeeze"><div class="right-corner"><div class="left-corner">
          <div class="breadcrumb"><a href="/">Home</a></div>

but here it is when viewed in IE8:

<li class="leaf last"><a href="/contact" title="Contact Plan UK using a form">Contact</a></li>
</ul></div>
</div>
        </div>
      
      <div id="center"><div id="squeeze"><div class="right-corner"><div class="left-corner">
          <div class="breadcrumb"><a href="/">Home</a></div>                              <h2>Your Floor Plan Is Here</h2> 

I've tried using a default theme just to check if that makes any difference, but it doesn't.

The site is at http://plan-uk.com/ - please could someone have a look and see if this behaviour is happening for them?

Any suggestions?

Thanks.