Hi David,
I have been meaning to get back to you with hopefully a fix for this, but couldn't just find the time. So better late than never!
Looking at the source code on your school's website using Firebug you get the following HTML markup...
<div id="inst2885" class="block_rss_client block">
<div class="header">
<div class="title">
<div class="block_action">
<h2>hillingdon.gov Twitter</h2>
<div class="commands"></div>
</div>
</div>
<div class="content">
<ul class="list no-overflow">
I've faded out the parts which don't apply as they're self contained and dont relate to the section that we need to set the css rule for.
So breaking this HTML down further we can derive a set of CSS id and class selectors to use to target the element we want to style.
And this is what we get... (Legend: #=id . = class)
div#inst2885 .block_rss_client .block div.content ul.no-overflow
Now if we add the { overflow: hidden;} to theend of that we get the CSS rule to fix the problem.
So putting this all together we get...
div#inst2885 .block_rss_client .block div.content ul.no-overflow {overflow: hidden;}
where this CSS only relates to that specific block element.
You can copy and paste this using Notepad and add it to the end of the same css file as before inplace if the previous css I gave you.
I have not tested this...but it should work. Remember to Purge all caches after you add the code, unless you have switched on Theme Designer Mode. Either way you need to make sure the cache is cleared before you refresh the screen to see the changes.
HTH
Mary