Thanks again for all your help guys.
Well, this proved to be a lot more difficult than I imagined it would be! Couldn't get most of your suggestions to work with folders, or couldn't understand the interface, e.g. BulkRenameUtility!
Others needed to be bought for sub-folders to work, etc. etc. I eventually got it done using Fast Folder Rename which is a weird bit of software that needs the files to be dropped into it to work. No real indication of that from the interface.
Anyway, it didn't handle sub-folders, so I had to drag and drop every set of folders into it from Explorer. Only about 150!
CommonTater: I don't use your suggested naming convention because it creates lots of folders with duplicate names, e.g. 16 for today, so searching is more tricky. But thanks for the suggestion anyway.