The dev console for page at https://deltaperformance.co.uk/blog.php?p=2 shows the following error -
Mixed Content: The page at 'https://deltaperformance.co.uk/blog.php?p=2' was loaded over HTTPS, but requested an insecure stylesheet 'http://fonts.googleapis.com/css?family=Yellowtail%7COpen%20Sans%3A400%2C300%2C600%2C700%2C800'. This request has been blocked; the content must be served over HTTPS.
As you can see, the font loading has been blocked because it is being invoked over plain http.
Please use 'https' with the fonts.googleapis.com URL and that should resolve the issue.
Hope this helps.