Hi,
In answer to my own question... ;-)
I created a new Smart Folder and went to 'other' in the first dropdown. I then selected 'Raw Query' and entered:
kMDItemFSName==.sit || kMDItemFSName==.zip || kMDItemFSName==.dmg || kMDItemFSName==.hqx || kMDItemFSName==.gz
This gives me any file with .sit, .zip etc in the name.
Simon