There is one thing in this game that I can't for the life of me work out, and that is how to buy a system from another race. I presume it is possible since there is an option for it under diplomacy but no matter what I do I can't get them to be click-on-able. Would someone please share with me the secret which has eluded me for so long now

You can only do so during the ceasefire.

I am not sure that ceasefire is related; this is just another name for a cold war state which is enforced for some number of turns. The AI considers systems to be very valuable and it will rarely offer them in trade. Normally the systems are greyed out in the trade menu, but you can hover the mouse over them to get some amusing quote. Occasionally, if an AI has a large number of systems and it can see you are beating it very badly, it may offer some, or allow you to click on them. But the AI has a strong notion of things which are too valuable to even offer in trade.

Ok well I assumed it was something like that, I guess I'll just have to wait for that rare occurrence in which I will be able to trade a system.