Thanks for your answers and advice.
I am simply using the server machine as a server, I am not using it as a general use machine, thats what the other ones are for. I seem to have it set up fine with its own domain etc, and all the machines seem to connect to it with no obvious problems.
To be honest I installed the server, not only to have a server in the house, but also to learn more about it. The best way to learn is to play with, although I best be careful, if I mess up the system again I am sure my missus will kill me
Next thing I would like to play with is a VPN. One of my machines is a laptop, and I will need to use it when I go away. I would like to be able to dial into my network and get my documents, connect to my e-mail folder etc. Any advice on a starting point for this?