These are basic ones - but what is the key to exit the game?? I can't find it in the manual and I have no choice but to alt control delete
AND
In the very first screen of the game, once i've interacted with an object I can't move tex around with the mouse anymore - only the keyboard?

In order to move Tex around with the mouse, you have to make sure that nothing is highlighted on the verb bar. If something IS highlighted, simply click the verb again to deselect it.
As for quitting the game, press CTRL and Q.

Thanks mate - perfect!

Actually, I had a hard time with Martian Memorandum myself.
i did eventually find that using the 'graphic mode setup" tool and selecting OpenGL took care of all the problems I'd had getting it to run. Wish it had occurred to me to try that before messing around with dosbox settings.

That's a new one to me, then, especially if martian memorandum is the only one you have a problem with.
You can always try alt+enter to force fullscreen.
I'd also check the dosbox console window to make sure it wasn't showing an error.
Other than that....I guess it's possible that MM's trying to use a full screen resolution your monitor won't support, but I don't know why it would be the only one doing it.
You might want to check the vogons thread on changing from the dosbox default dynamic resolution to a fixed resolution and see if that helps.
Info's here: http://vogons.zetafleet.com/viewtopic.php?t=9498

I seem to have a strange problem with the game. It automatically passes the text information, as if autoclicking, and I don't have time to read it. It didn't behave like that at the beginning, did I turn an option on by mistake ?