Very simple forum-like behaviour can be implemented using the built-in 'conversation' block.

Probably the most thorough c5-based solution is
https://www.concrete5.org/community/forums/customizing_c5/interest-i...
It's still in development and isn't as easy to install as marketplace add-ons, but it seems to be getting positive feedback.

phpBB and the like are substantially more powerful, but getting the same look and feel across your site would be nearly impossible.

If you can do a bit of fiddling, there ishttps://github.com/ortic/concrete5-forum...

Thanks very much I am going to go with phpBB.

Michael.