Yes, it's strange. I have reverted back to verson of DB before running the query and just ran it again and table is now correct with correct (195662) number of records.
Not sure what happened, possibly a bug as it was running first time but it seems to be working now thanks.
Just conscious next time I run it with more data, is it going to reconvert the converted ItemIDs by adding an extra 'EZ' in front of them (so I then get EZEZ1001 for instance)? What would I need to do to stop this?
Sorry for so many questions!!