[support] caching database credentials

Hi

I have an odd issue which i hope someone can help resolve. In test my sites were pointing at one db server and now they are on a host where they are pointing at a db on localhost. the sites work fine for adding/viewing content etc (for the last 5 days), but when i come to change the theme, or i change a users role, i get a screen saying that the site could not connect to the OLD database. I have checked settings.php (im using multisites, have grepped everything).

im assuming this is cached in the db somewhere, but wondered if there was a function to clean out the db cache (have used empty cache many times, and am running with cache disabled) before i start deleting stuff randomly...

any help appreciated.