Many thanks for the replies. I now have the DocumentLibrary product installed, as recommended by a few people here.

I have come across one oddity with it so far though. For some reason, the 'approval' process only works if we select the 'Add Document' button from within the 'Documents' folder (the default document store that is included inside every new 'document library'). If we go via the default document_submit DTML method, then all docs are immediately made available, bypassing the approval stage completely.