If I have logged into X on my desktop computer, is there anyway Ican "look-in" on the session on my laptop? I can remotely run things on my desktop PC to be displayed and used on mylaptop but I want to be able to see and preferably interact with things started on the desktop PC.

You can use VNC. See:
http://www.realvnc.com/v4/x0.html

The X system is built using a client/server architecture making it possible for a program to run on one computer but connect to another computer to show it's output there. This would not show you the desktop of your laptop. It would look like the program is run locally on your computer.

This can be set up in a few ways, ranging from insecure to highly secure.
One way is to use the function built into X, this is insecure and a bit tricky.
The other solution is to use ssh to forward X graphic through an encrypted tunnel, which is both secure and quite easy to set up.

I'll look into the VNC, as I think this might be what I am after.
I think, Ugge, you may have misunderstood my badly worded question
At the moment I do startx on my laptop and it starts with xterm rather than a window manager and I do :
ssh -XC 192.168.1.1
and start everything remotely, via my PLIP connection (things run on my desktop PC and are displayed on my laptop ). What I wanted was to be able to look-in on a X session already running on the desktop computer.

Thank-you

I cant remember exactly how to do it but try using

screen

with screen you can start programs running, then disconnect from them, shutdown your remote connection, comeback later and reconnect and continue what you are doing.

try searching the forums and you should find some info.