The message I get in chrome says:
melfordschoolofmotoring.co.uk contains content from 220.127.116.11, a
site known to distribute malware. Your computer might catch a virus if
you visit this site.
If you look at your website's headers using livehttp headers (see note below about how I found this code) you'll see a request for
http://18.104.22.168/s.php which is probably what is inserting the code below into the head of your page before the dtd:-
It's stealthy because I couldn't see this code in chrome or an agent that looks at the page as if it's the googlebot, only in a private browsing session in firefox.
I would check your include files, htaccess files and root directory very carefully to look for signs of a hack. I would also check your file and directory permissions are 755 for directories and 644 for files.
Beyond that consider some security software like website defender and signing up for webmaster tools. Once you've found and cleaned up the infection submit a reconsideration request to google via the webmaster tools account.
I could just be the hack, but your site's performance is pretty bad, it may be worth considering a new host as page load speed is a factor for google and I can't see why some simple php includes would take so long to load. Keep an eye on performance once you've cleaned up.