Hi
OK I'm an idiot!
It's actually a problem with Firefox (v2.0.0.14) caching.
If you change the captcha type in the Admin-Tool Firefox does not display the captcha at all on the Feedback page until you completely clear the cache - even a Ctrl+F5 page refresh which is supposed to force the content to be pulled again from the server - doesn't help. Clear all cached files, restart browser and then revisit the page and it works fine.
I really am quite embarrassed! Apologies for wasting people's time

Thanks so much!

Graham