Turns out the full-text search funtionality of our Windows Sharepoint Services installation has not been working since I moved the content databases to a new SQL server over Christmas.  After digging into some doco. on the Microsoft Search service (which is the full text search service's real name) I fluked the way to fix it!  All you need do is goto the Sharepoint Central Administration interface, turn off Full Text Indexing and turn it back on again.  Upon turning it on again it creates a new catalogue and starts to populate it.  Easy!