Intercept no permissions message

[rob30UK]

I have a specific forum where you must be a member of a specific public group in order to gain access...

I want the forum visible however so that when people click on it, instead of beinbg presented with a no permissions page, they get a nice message saying they need to join X group in order to access.

Could some kind sould point me in the direction of an existing hack, or per chance tell me how to intercept the no permissions on a per forum basis.

There are a two forums requiring different group access, and different messages need to be displayed for each when they try and access the forum.

I would be most grateful for someone pushing me along the right path.