In Your case it's hacked site or You have some component or module that is showing seo links (it's best to delete it if You find out which one does it).
Try to seach in content of files (total commander can do this via ftp) for "Bookmakers bonuses", "gbetting.co.uk" or "-6500px". If You find the problematic file You can find the date of file change. If so it's best to take clean files from backup if it's possible.
If code is placed in module or component dir it's big chanse that it's not a hack attempt but component or module from spam developer.
You can also scan your site with:
http://virustotal.com - many virus scanners. It's best place to start.
http://quttera.com - it shows which files are corupted
sitecheck.sucuri.net - similar to above
www.urlvoid.com - similar to above
If any of these sites show infection, You have to clean Youre site.