OK. Looks like I HAVE A SOLUTION to my problem now!
Please see this thread:
As you can probably tell from the posts I am not a network expert, so apologies if the below is not a very technical way of explaining what I did.
It looks like in the end it probably was some sort of address assigning conflict.
I set up a local VLAN subnet with the VPN server running on 10.0.0.1 as documented at http://macminicolo.net/mountainlionvpn
Now clients logging in via VPN get 10.0.0.xxx addresses instead of the previous 192.168.1.xxx
You MUST set up Internet Routing as described, which involves a few lines in terminal and text edit. (http://macminicolo.net/mountainlionvpn)
This has in the past very frustratingly resulted in a functioning VPN connection but no internet connection for the VPN clients.
Maybe it has something to do with the fact that I am running a Squid caching proxy on the same macine that is also running the VPN server. So I set Squid to provide proxy services for the 10.0.0.0 subnet . . .
And now it all works : )
Not sure if it would have worked without Squid, I did not try to uninstall the Proxy. I am guessing it wouldn't because when you restart the machine, the VPN service works, but you only get internet access once the administrator logs in and Squid starts.
If it is required, you can install Squid with a reasonable GUI via SquidMan http://squidman.net/squidman/
My last question would now be - can you set it up somehow that Squid gets launched BEFORE someone logs in (i.e. not as a login item), so that if there is a power failure and the computer restarts, it restarts Squid as well, regardless of someone logging in or not?
Hope this helps some people out there : )