Normally this is handled automatically by WordPress but I would check a few things to make sure:
- When you type of old URL are you redirected to the new one? Or do you see the same content via two URLs?
- If you are redirected, make sure that the redirect is a 301 Permanant One.
If you are not redirected, the page needs a canonical tag to tell Google which is the official URL. Look at the generated source code to see if you find a:
<link rel="canonical" href="http://www.thecheckeredflag.co.uk/mtech-lite-retain-barc-regulars-for-winter-series/">
If the canonical is missing and you are not redirected, you have something to fix.
Now, if Wordpress handled it correctly, why is it broken? you may ask. The answer is that Google just did not get around to seeing all you pages with the new structure and seeing the old ones (with a redirect or canonical) and updating their index. This just takes time and many be a few weeks to even months, depending on the size and crawling frequency on your site.
The bottom line is that you should only worry about duplicates if you actually have some, which in your case is not true.