Skyscraper ad will not display properly

Hi everyone,

I put a skyscraper ad on the sidebar of my blog for the first time today, however the full length of the ad will not display. You can see what I mean by visiting my blog and scrolling to the bottom of the sidebar.

The ad sponsor has assured me that the code is correct and it must be related to my theme. Any ideas? Even when I tried the AdRotate plugin, which the sponsor suggested it made no difference which tells me it must be the theme?

Thanks so much for your help.
https://www.smittenbybritain.com