Can someone give a little guidance please?

I think the reason you're not getting many answers is that, so far as I know, there aren't really many guidelines. The mods give us a lot of free reign so long as we stay within the game. I think it's fair to ask for a cancel, but it's also not something you should consider a given. If someone doesn't want to, form an alliance against them so that the situation no longer benefits them. Other than that, hope you're playing with people who want a challenge instead of easy points. Most people I've played with tend to be reasonable here.

Thanks for the reply and I think you are right, but unfortunately I am in a game with at least one person who isn't that reasonable

Well, it alway depends on the situation.
I personally do not think it is unfair if someone who has an advantage does not cancel.(maybe not "fair play" but not unfair in the sense of cheating) Since he probably has in the past lost because of having a disadvantage. So all in all it should be even over longterm.
Of course I personally would cancel if everyone else want to cancel. Say I am Germany and France AND En CD, then if everyone wants to cancel I will too.
Just really depends on the situation.

Of course if someone does not want to cancel or draw everybody can just turn on him(which I think is fair game), often then the person will change his mind.

As you said, it can be hard to get a cancel even when you would argue it should happen.
Can anyone think of a time when you'd get two CDs in 1901 and *not* yet feel the game was fair to play on? Perhaps that could be automated such that the 2nd one cancels the game

I was playing austria I think and Russia and Turkey weren't there...felt like Christmas :P...but yes I lost as it was practically WW2 (i.e. everyone and their kid sister ganged up against me).

figlesquidge: yes
for example if england and turkey go cd (only italy has an unfair advantage, but i always has, and if moves to get some of turkey it can still win)
austria and england cding is even better

I guess it does make for a fair varient, as long as everyone knows they won't be there - its if others play assuming the nations are present and they're not you get imbalances. I stand corrected.