callback url not being processed

Hi Team - I'm a novice so please be gentle!

I am helping maintain a CI 2.2 installed website and am trying to get a callback working from a 3rd party (Webpurify)

I can see the response in the apache2 log (running on Ubuntu), and I have added a route so that the function parameter is correctly interpreted.

However, the code (php) is never called.

If I copy the callback URL and paste it into my browser, it works! (ie the post processing code works fine and I get a DB update etc.)

I suspect .htaccess but if it works from my browser I'm not convinced.

I also noticed in the apache2 log that when the http GET data is suffixed by a brwoser list it seems to work and when it is suffixed by "Webpurify(callback)" it fails. Is there somewhere in CI I need to tel it to accept this callback?

Any help greatly appreciated,

Thanks, Paul