Thanks for everybodys helpful response.

One thing I have also noticed is we did not have a custom 404 page up i.e. a zencart one with the sitemap. So instead it was linking to a server 404 error document. Im not sure if that was causing problems with the SQL database.

Anyway I've blocked the IP range for the time being. Im a bit dubious of blocking the whole of China, even though 99% are unwelcome visitors. I do not want it to effect google rankings in other Asia countries.