Got a hit yesterday from 31.40.253.189. BGP lookup shows that it's part of 31.40.252.0/23 (Blockchain Network Solutions Ltd).
A little more digging for Blockchain Network gives this:
31.40.252.0/23
213.166.78.0/23
213.166.76.0/23
2.57.76.0/23 (AS43444)
176.119.141.0/24 (AS43444)
Two of those are listed as being in the UK, the other 3 in Russia. The hit was directly to a pdf file on my site, but with some extra junk added on the end:
/some-pdf-file.pdf/phpAds/click.php3
That generated a 404 response from my server. The entire user-agent was:
Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)
I'm now going to be adding all the above CIDR's to my router's blocking list. That list now consists of almost 28,000 CIDR's comprising about 582 million usable IPv4 addresses.