Is there any way to block certain sites, eg MacKeeper? MacKeeper (and other sites) adverts pop up on some video sites which I find very irritating. Is there any way to block specific URLs in Safari?

This occurs both on Mac and iPad. Ideally, I'd like a solution I can directly apply to the iPad.
I would suggest OpenDNS.  You can apply it to your entire network using their name servers and block categories or specific URL's.  Although the ad might show up as a frame to a 'Website Blocked' page from OpenDNS, you'll probably find it useful (it's free).