Thanks for the welcome, much appreciated. Totally unforgiveable for not posting here before.
I have hunted round both places and nothing is quite there just yet!
I have done a major search across the net regarding forums, content mgt systems, etc and I can't find one that has the feature of being able to use a different component for users mgt.
Looks like we are going to have to create it. When we do it will be OSS, so will let you know.
The main reason for the requirement is that often a site user has to register twice on a site - once for the main site and secondly for any associated forum.
I also came across a rather good ecommerece system called osCommerce, but again it has its own user mgt system which could potentially result in a user having to login a 3rd time on the same site.
With so much of this type of software eveolving, I would be very interested if anyone comes aacross any kind of component that allows cross site/module functionality.